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Solar Flare

Solar flare

] A solar flare is a violent explosion in the Sun's atmosphere with an energy equivalent to tens of millions of hydrogen bombs. Solar flares take place in the solar corona and chromosphere, heating plasma to tens of millions of kelvins and accelerating the resulting electrons, protons and heavier ions to near the speed of light. They produce electromagnetic radiation across the electromagnetic spectrum at all wavelengths from long-wave radio to the shortest wavelength gamma rays. Solar flares were first observed on the Sun in 1859 by English astronomer Richard Carrington. Similar stellar flares have also been observed to varying degrees on other stars in modern times. The frequency of solar flares varies, from several per day when the Sun is particularly "active" to less than one each week when the Sun is "quiet". Solar flares may take several hours or even days to build up, but the actual flare takes only a matter of minutes to release its energy.

Classification of flares

Solar activity is classified as A, B, C, M or X according to the irradiance of its X-rays near Earth as measured on the GOES spacecraft in watts per square meter (W/m2). Each class is ten times more powerful than the preceding one, with X at 104 W/m2). Within a class there is a linear scale from 1 to 9, so an X2 flare (twice as powerful as an X1 flare) is four times more powerful than an M5 flare (five times as powerful as an M1 flare). Solar activity is normally within the A to C range. Class C flares have little effect on Earth, while the more powerful M and X flares can cause disruption and damage. X flares are the most powerful, displaying the highest level of strength. Flares generally stay below X10, but infrequently X designations run 'off the charts'. X20 events (2 mW/m2) that were recorded on August 16, 1989 and April 2, 2001 were outshone by a flare on November 4, 2003 that was the most powerful X-ray flare ever recorded, which was originally thought to be an X28 (2.8 mW/m2). The data is unclear because the detection stystems were overloaded with all kinds of electromagnetic radiation prior to the peak, but it is now thought that the flare was between an X40 (4.0 mW/m2) and an X45 (4.5 mW/m2). The scientists report can be found here http://www.agu.org/pubs/crossref/2005/2004JA010960.shtml. Sunspot Region 486, where this flare originated (shown in the illustration above several days before the eruption), was the most turbulently active sunspot ever recorded. The most powerful flare of the last 500 years was believed to have occurred in September 1859: it was seen by British astronomer Richard Carrington and left a trace in Greenland ice in the form of nitrates and beryllium-10, which allow its strength to be measured today. (New Scientist, 2005)

Hazards

beryllium throughout the Solar System. Over 99.999% of the interplanetary medium by volume is plasma. http://antwrp.gsfc.nasa.gov/apod/ap020516.html Ref & Credit ]] It was long thought that solar flares send out streams of highly energetic solar wind that can present a radiation hazard to spacecraft outside of a planetary magnetosphere and can disrupt radio signals on Earth. They were also thought to be a primary contributor to the aurora borealis and aurora australis and to Solar proton events. However, it is now thought that Coronal Mass Ejections (CMEs), which frequently accompany flares, are the main cause of such effects on and around the Earth. Solar flares release a cascade of high energy particles known as a proton storm. Protons can pass through the human body, doing biochemical damage. Most proton storms take two or more hours from the time of visual detection to reach Earth. A solar flare on January 20, 2005 released the highest concentration of protons ever directly measured, taking only 15 minutes after observation to reach Earth. [http://www.example.com link title] The radiation risk posed by solar flares and CMEs is one of the major concerns in discussions of manned missions to Mars or to the moon. Some kind of physical or magnetic shielding would be required to protect the astronauts. Originally it was thought that astronauts would have two hours time to get into shelter. Based on the January 20 event, they may have as little as 15 minutes to do so.

See also


- Coronal mass ejection
- Geomagnetic storm

References


-
- Mewaldt, R.A., et al. 2005. Space weather implications of the 20 January 2005 solar energetic particle event. Joint meeting of the American Geophysical Union and the Solar Physics Division of the American Astronomical Society. May 23-27. New Orleans. [http://www.agu.org/cgi-bin/SFgate/SFgate?language=English&verbose=0&listenv=table&application=sm05&convert=&converthl=&refinequery=&formintern=&formextern=&transquery=an%3d%22SH32A-05%22&_lines=&multiple=0&descriptor=%2fdata%2fepubs%2fwais%2findexes%2fsm05%2fsm05%7c999%7c4995%7cSpace%20Weather%20Implications%20of%20the%2020%20January%202005%20Solar%20Energetic%20Particle%20Event%7cHTML%7clocalhost:0%7c%2fdata%2fepubs%2fwais%2findexes%2fsm05%2fsm05%7c7840533%207845528%20%2fdata2%2fepubs%2fwais%2fdata%2fsm05%2fsm05.txt Abstract].
- [http://www.solcomhouse.com/solar.htm The Sun]

External links


- [http://news.bbc.co.uk/1/hi/sci/tech/3251481.stm BBC report on the November 4, 2003 flare]
- [http://soho.nascom.nasa.gov/hotshots/ NASA SOHO observations of flares]
- [http://www.ucm.es/info/Astrof/invest/actividad/flares.html Stellar Flares] - D. Montes, UCM.
- [http://www.ucm.es/info/Astrof/obs_ucm/sol/sol.html The Sun] - D. Montes, UCM. ja:太陽フレア Flare Category:Stellar phenomena Category:Space plasmas Category:Plasma physics

Energy

Energy is a measure of being able to do work. This is a fundamental concept pertaining to the ability for action. In physics, it is a quantity that every physical system possesses. This quantity is not absolute but relative to a state of the system known as its reference state or reference level. The energy of a physical system is defined as the amount of mechanical work that the system can produce if it changes its state to its reference state; for example if a liter of water cools down to 0°C or if a car hits a tree and decelerates from 120 km/h to 0 km/h. Energy of an object can be in several forms, potential—due to the position of the object relative to other objects; kinetic—energy because of its motion; chemical—due to chemical bonds between atoms that make up the substance; electrical—due to its charge; thermal—due to its heat; and nuclear—due to the instability of the nuclei of its atoms. In the case where the "object" is an electromagnetic wave or light, then radiant energy can also be defined. One form of energy can be readily transformed into another; for instance, a battery converts chemical energy into electrical energy, which can be converted into thermal energy. Similarly, potential energy is converted into kinetic energy of moving water and turbine in a dam, which in turn transforms into electric energy by generator. The law of conservation of energy states that in a closed system the total amount of energy, corresponding to the sum of a system's constituent energy components, remains constant. This law follows from translational symmetry of time (that is, independence of any physical process on the moment it started). Some works (thus some forms of energy) are not easily measured by the unaided observer. The term 'energy' is also used in a spiritual or non-scientific way that cannot be quantified, to make certain prepositions look like they are more plausible, by imitating the scientific terminology. Usually this has something to do with mystical and/or healing type references such as acupuncture and reiki.

Forms of Energy

Below is a list of different energy forms. Lotka (1956, p. 5) asked an interesting question about what defines an energy form. :"We are equipped with two separate and distinct senses, the one responding to electromagnetic waves ranging from about 4×10-4 to 8×10-4 mm., light waves; the other to somewhat longer waves otherwise of the same character, heat waves. Accordingly we have two separate terms in our language light and heat, to denote two phenomena which, objectively considered, are not separated by any line of division, but merge into one another by gradual transition. Here the question might be raised whether an electromagnetic wave of a length of 9×10-4 mm. is a light wave or a heat wave." That is to ask, if all forms of energy are defined in terms of infinitesimal increments of the wave spectrum, what makes one form of energy different to another?
- Kinetic energy: the energy of moving objects
  - Thermal energy: the energy associated with heat
  - Sound energy: the energy of compression waves
  - Electrical energy: the energy of moving charged particles
- Potential Energy: the energy that an object has due to position; also known as stored energy
  - Chemical energy: the stored energy of chemical substances
  - Nuclear energy: the stored energy of the atomic nucleus
- Radiant energy: the energy of electromagnetic waves, including light

Units

SI

The SI unit for both energy and work is the joule (J), named in honour of James Prescott Joule and his experiments on the mechanical equivalent of heat. In slightly more fundamental terms, 1 joule is equal to 1 newton-metre and, in terms of SI base units: 1\ \mathrm = 1\ \mathrm \left( \frac \right ) ^ 2 = 1\ \frac An energy unit that is used in particle physics is the electronvolt (eV). One eV  is equivalent to 1.60217653×10−19 J. In spectroscopy the unit cm-1 = 0.0001239 eV is used to represent energy since energy is inversely proportional to wavelength from the equation E = h \nu = h c/\lambda . (Note that torque, which is typically expressed in newton-metres, has the same dimension and this is not a simple coincidence: a torque of 1 newton-metre applied on 1 radian requires exactly 1 newton-metre=joule of energy.)

Other units of energy

In cgs units, one erg is 1 g cm2 s−2, equal to 1.0×10−7 J. Another obsolete metric unit is the litre-atmosphere (101.325 J). The imperial/US units for both energy and work include the foot-pound force (1.3558 J), the British thermal unit (Btu) which has various values in the region of 1055 J, and the horsepower-hour (2.6845 MJ). The energy unit used for everyday electricity, particularly for utility bills, is the kilowatt-hour (kW h), and one kW h is equivalent to 3.6×106 J  (3600 kJ or 3.6 MJ; the metric units usually are self-consistent, and this particular one may seem arbitrary; it's not, the metric measurement for time is the second, and there are 3,600 seconds in an hour -- in other words, 1 kW second = 1 kJ, but the kW h is a more convenient unit for everyday use). The calorie is mainly used in nutrition and equals the amount of heat necessary to raise the temperature of one kilogram of water by 1 degree Celsius, at a pressure of 1 atm. This amount of heat depends somewhat on the initial temperature of the water, which results in various different units sharing the name of "calorie" but having slightly different energy values. It is equal to 4.1868 kJ. The calories used for food energy in nutrition are the large calories based on the kilogram rather than the gram, often identified as food calories. These are sometimes called kilocalories with that calorie being the small calorie based on the gram, and as a result the prefixes are generally avoided for the large calories (i.e., 1 kcal is 4.184 kJ, never 4.184 MJ, even if "calories" are also used for the other, larger unit in the same document or the same nutrition label). Food calories are sometimes noted as Calories (1000 calories) or simply abbreviated Cal with the capital C, but that convention is more often found in chemistry or physics textbooks—which do not use these large calories—than it is in real-world applications by those who do use these calories. (This convention is also, of course, useless when the word calorie appears in a location where it would ordinarily be capitalized, as at the beginning of a sentence or in the first column of a nutrition label as a substitute for the quantity being measured, which is energy, when all the other quantities such as "Iron" and "Sugars" are also capitalized.)

Transfer of energy

Work

Main article: mechanical work. Work is a defined as a path integral of force F over distance s: : W = \int \mathbf \cdot \mathrm\mathbf The equation above says that the work (W) is equal to the integral of the dot product of the force (\mathbf) on a body and the infinitesimal of the body's position (\mathbf).

Heat

Main article: Heat. Heat is the common name for thermal energy of an object that is due to the motion of the atoms and molecules that constitute the object. This motion can be translational (motion of molecules or atoms as a whole; vibrational - relative motion of atoms within molecules or rotational motion. It is the form of energy which is usually linked with a change in temperature or in a change in phase of matter. In chemistry, heat is the amount of energy which is absorbed or released when atoms are rearranged between various molecules by a chemical reaction. The relationship between heat and energy is similar to that between work and energy. Heat flows from areas of high temperature to areas of low temperature. All objects (matter) have a certain amount of internal energy that is related to the random motion of their atoms or molecules. This internal energy is directly proportional to the temperature of the object. When two bodies of different temperature come in to thermal contact, they will exchange internal energy until the temperature is equalised. The amount of energy transferred is the amount of heat exchanged. It is a common misconception to confuse heat with internal energy, but there is a difference: the change of the internal energy is the heat that flows from the surroundings into the system plus the work performed by the surroundings on the system. Heat Energy is transferred in three different ways: conduction, convection and/or radiation.

Conservation of energy

The first law of thermodynamics says that the total inflow of energy into a system must equal the total outflow of energy from the system, plus the change in the energy contained within the system. This law is used in all branches of physics, but frequently violated by quantum mechanics (see off shell). Noether's theorem relates the conservation of energy to the time invariance of physical laws. An example of the conversion and conservation of energy is a pendulum. At its highest points the kinetic energy is zero and the potential gravitational energy is at its maximum. At its lowest point the kinetic energy is at its maximum and is equal to the decrease of potential energy. If one unrealistically assumes that there is no friction, the energy will be conserved and the pendulum will continue swinging forever. (In practice, available energy is never perfectly conserved when a system changes state; otherwise, the creation of perpetual motion machines would be possible.) Another example is a chemical explosion in which potential chemical energy is converted to kinetic energy and heat in a very short time.

Types of energy

All forms of energy: thermal, chemical, electrical, radiant, nuclear etc. can be in fact reduced to kinetic energy or potential energy. For example thermal energy is essentially kinetic energy of atoms and molecules; chemical energy can be visualized to be the potential energy of atoms within molecules; electrical energy can be visualized to be the potential and kinetic energy of electrons; similarly nuclear energy is the potential energy of nucleons in atomic nucleii.

Kinetic energy

Main article: Kinetic energy. Kinetic energy is the portion of energy related to the motion. :E_k = \int \mathbf \cdot \mathrm\mathbf The equation above says that the kinetic energy (E_k) is equal to the integral of the dot product of the velocity (\mathbf) of a body and the infinitesimal of the body's momentum (\mathbf). For non-relativistic velocities, that is velocities much smaller than the speed of light, we can use the Newtonian approximation :E_k = \begin \frac \end mv^2 where Ek is kinetic energy m is mass of the body v is velocity of the body At near-light velocities, we use the correct relativistic formula: :E_k = m c^2 (\gamma - 1) = \gamma m c^2 - m c^2 \;\! :\gamma = \frac where v is the velocity of the body m is its rest mass c is the speed of light in a vacuum, which is approximately 300,000 kilometers per second \gamma m c^2 \, is the total energy of the body m c^2 \, is again the rest mass energy. See also, E=mc². In the form of a Taylor series, the relativistic formula for can be written as: :E_k = \frac mv^2 - \frac \frac + \cdots Hence, the second and higher terms in the series correspond with the "inaccuracy" of the Newtonian approximation for kinetic energy in relation to the relativistic formula. However, the phrase "conservation of energy" is often confusing to a non scientist. This is so, because of the common usage of the terms "save energy" or conserve energy" used in campaigns for conservation of energy resources like electricity or fossil fuels.

Potential energy

Main article: Potential energy. In contrast to kinetic energy, which is the energy of a system due to its motion, or the internal motion of its particles, the potential energy of a system is the energy associated with the spatial configuration of its components and their interaction with each other. Any number of particles which exert forces on each other automatically constitute a system with potential energy. Such forces, for example, may arise from electrostatic interaction (see Coulomb's law), or gravity. In an isolated system consisting of two stationary objects that exert a force f(x) on each other and lay on the x-axis, their potential energy is most generally defined as :E_p = -\int f(x) \, dx where the force between the objects varies only with distance x and is integrated along the line connecting the two objects. To further illustrate the relationship between force and potential energy, consider the same system of two objects situated along the x-axis. If the potential energy due to one of the objects at any point x is U(x), then the force on the that object x is :f(x) = -\frac This mathematical relationship demonstrates the direct connection between force and potential energy: the force between two objects is in the direction of decreasing potential energy, and the magnitude of the force is proportional to the extent to which potential energy decreases. A large force is associated with a large decrease in potential energy, while a small force is associated with a small decrease in potential energy. Notice how, in this case, the force on an object depends entirely on its potential energy. These two relationships – the definition of potential energy based on force, and the dependence of force on potential energy – show how the concepts of force and potential energy are intimately linked: if two objects do not exert forces on each other, there is no potential energy between them. If two objects do exert forces on each other, then potential energy naturally arises in the system as part of the system's total energy. Since potential energy arises from forces, any change in the system's spatial configuration will either increase or decrease the system's potential energy as the objects are repositioned. When a system moves to a lower potential energy state, energy is either released in some form or converted into another form of energy, such as kinetic energy. The potential energy can be "stored" as gravitational energy, elastic energy, chemical energy, rest mass energy or electrical energy, but arises in all cases from the spatial positioning and interaction of objects within a system. Unlike kinetic energy, which exists in any moving body, potential energy exists in any body which is interacting with another object. For example a mass released above the Earth initially has potential energy resulting from the gravitational attraction of the Earth, which is transferred to kinetic energy as the gravitational force acts on the object and its potential energy is decreased as it falls. Equation: :E_p = mgh \; where m is the mass, h is the height and g is the value of acceleration due to gravity at the Earth's surface (see gee).

Internal energy

Main article: Internal energy. Internal energy is the kinetic energy associated with the motion of molecules, and the potential energy associated with the rotational, vibrational and electric energy of atoms within molecules. Internal energy, like energy, is a quantifiable state function of a system.

History

In the past, energy was discussed in terms of easily observable effects it has on the properties of objects or changes in state of various systems. Basically, if something changed, some sort of energy was involved in that change. As it was realized that energy could be stored in objects, the concept of energy came to embrace the idea of the potential for change as well as change itself. Such effects (both potential and realized) come in many different forms; examples are the electrical energy stored in a battery, the chemical energy stored in a piece of food, the thermal energy of a water heater, or the kinetic energy of a moving train. To simply say energy is "change or the potential for change", however, misses many important examples of energy as it exists in the physical world. The concept of energy and work are relatively new additions to the physicist’s toolbox. Neither Galileo nor Newton made any contributions to the theoretical model of energy, and it was not until the middle of the 19th century that these concepts were introduced. The development of steam engines required engineers to develop concepts and formulas that would allow them to describe the mechanical and thermal efficiencies of their systems. Engineers such as Sadi Carnot and James Prescott Joule, mathematicians such as Émile Claperyon and Hermann von Helmholtz , and amateurs such as Julius Robert von Mayer all contibuted to the notions that the ability to perform certain tasks, called work, was somehow related to the amount of energy in the system. The nature of energy was elusive, however, and it was argued for some years whether energy was a substance (the caloric) or merely a physical quantity, such as momentum. William Thomson (Lord Kelvin) amalgamated all of these laws into his laws of thermodynamics, which aided in the rapid development of energetic descriptions of chemical processes by Rudolf Clausius, Josiah Willard Gibbs, Walther Nernst. In addition, this allowed Ludwig Boltzmann to describe entropy in mathematical terms, and to discuss, along with Jožef Stefan, the laws of radiant energy. For further information, see the Timeline of thermodynamics.

Energy and Economy

Main articles: energy development, energy policy The way in which humans use energy is one of the defining characteristics of an economy. The progression from animal power to steam power, then the internal combustion engine and electricity, are key elements in the development of modern civilization. Future energy development, for example of renewable energy, may be key to avoiding the effects of global warming.

See also


- Principles of energetics

Energy in natural sciences


- Energy conversion
- Enthalpy
- Exergy
- Power (physics)
- Specific orbital energy
- Solar radiation
- Thermodynamics
- Thermodynamic entropy

Energy resources


- List
- Embodied energy
- Emergy
- Crisis
- Development
- Policy
- Renewable
- Energy balance
- Management
- Storage
- Transmission
- EU Energy Label
- EU Intelligent Energy,
- Efficiency

Other energy Topics

Links to the miscellaneous topics related to energy
- A list of other topics and their links

Further reading


- Feynman, Richard. Six Easy Pieces: Essentials of Physics Explained by Its Most Brilliant Teacher. Helix Book. See the chapter "conservation of energy" for Feynman's explanation of what energy is and how to think about it.
- Einstein, Albert (1952). Relativity: The Special and the General Theory (Fifteenth Edition). ISBN 0-517-88441-0
- Alfred J. Lotka (1956). Elements of Mathematical Biology, forerly published as 'Elements of Physical Biology', Dover, New York.

Notes

This definition is one of the most common; e.g. [http://observe.arc.nasa.gov/nasa/space/stellardeath/stellardeath_6.html Glossary at the NASA homepage]

External links


- [http://www.unitconversion.org/unit_converter/energy.html Online Energy and Work Converter] - convert between various units of energy and work, such as joule, erg, gigawatt-hour, newton meter, calorie, Btu, and so on
- [http://www.unitconversion.org/unit_converter/energy-v.html Interactive Energy and Work Conversion Table] - convert selected unit to all other units of energy and work
- [http://jumk.de/calc/energy.shtml Conversions of energy units]
- [http://www.physicsweb.org/article/world/15/7/2 What does energy really mean? From Physics World]
- [http://www.energy.ca.gov/glossary/ Glossary of Energy Terms]
- [http://www.iea.org International Energy Agency IEA - OECD] Category:Introductory physics Category:Fundamental physics concepts Category:Physical quantity ko:에너지 ms:Tenaga ja:エネルギー simple:Energy th:พลังงาน

Hydrogen bomb

Nuclear weapon

Chromosphere

The chromosphere (literally, "color sphere") is a thin layer of the Sun's atmosphere just above the photosphere, roughly 10,000 kilometers deep (approximating to, if a little less than, the diameter of the Earth). The chromosphere is more visually transparent than the photosphere. Without special equipment the Sun's chromosphere cannot be seen due to its being washed out by the overwhelming brightness of the photosphere, but its reddish colour can be seen during a total solar eclipse or in filtered light such as H-alpha. H-alpha eclipse]] The most common solar feature within the chromosphere are Spicules, long thin fingers of luminous gas which appear like the blades of a huge field of fiery grass growing upwards from the photosphere below. Spicules rise to the top of the chromosphere and then sink back down again over the course of about 10 minutes. Another feature found in the chromosphere are fibrils, horizontal wisps of gas similar in extent to spicules but with about twice the duration. Finally, solar prominences rise up through the chromosphere from the photosphere, sometimes reaching altitudes of 150,000 kilometers. These gigantic plumes of gas are the most spectacular of solar phenomena, aside from the less frequent solar flares. Above the chromosphere of some stars there is a so-called transition region, where the temperature increases rapidly to the hot corona, which forms the outermost part of the atmosphere. Category:Sun ja:彩層

Kelvin

The kelvin (symbol: K) is the SI unit of temperature, and is one of the seven SI base units. It is defined as the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. A temperature given in kelvins, without further qualification, is measured with respect to absolute zero, where molecular motion stops. It is also common to give a temperature relative to the reference temperature of 273.15 K, approximately the melting point of water under ordinary conditions; this convention is the Celsius temperature scale. The kelvin is named after the British physicist and engineer William Thomson, 1st Baron Kelvin; his barony was in turn named after the River Kelvin, which runs through the grounds of the University of Glasgow.

SI multiples

Typographical conventions

The word kelvin as an SI unit is correctly written with a lowercase k (unless at the beginning of a sentence), and is never preceded by the words degree or degrees, or the symbol °, unlike degrees Fahrenheit, or degrees Celsius. This is because the latter are adjectives, whereas kelvin is a noun. It takes the normal plural form by adding an s in English: kelvins. When the kelvin was introduced in 1954 (10th General Conference on Weights and Measures (CGPM), Resolution 3, CR 79), it was the "degree Kelvin", and written °K; the "degree" was dropped in 1967 (13th CGPM, Resolution 3, CR 104). Note that the symbol for the kelvin unit is always a capital K and never italicised. There is a space between the number and the K, as with all other SI units. Unicode includes the "kelvin sign" at U+212A (in your browser it looks like K). However, the "kelvin sign" is canonically decomposed into U+004B, thereby seen as a (preexisting) encoding mistake, and it is better to use U+004B (K) directly.

Conversion factors

Kelvins and Celsius

The Celsius temperature scale is now defined in terms of the kelvin, with 0 °C corresponding to 273.15 kelvins.
- kelvins to degrees Celsius
- : \mathrm = \mathrm - 273.15

Temperature and energy

In a thermodynamic system, the energy of the particles of a perfect gas is proportional to the absolute temperature, where the constant of proportionality is the Boltzmann constant. As a result, it is possible to determine the average kinetic energy \overline of the gas particles at the temperature T or to calculate the temperature of the gas from the average kinetic energy of the particles: : \overline = \frac \cdot k_B \cdot \mathrm

See also


- ITS-90 International Temperature Scale

External link


- [http://www1.bipm.org/en/si/si_brochure/chapter2/2-1/2-1-1/kelvin.html BIPM brochure on the kelvin] Category:SI base units Category:Units of temperature ko:켈빈 ja:ケルビン simple:Kelvin th:เคลวิน

Proton

:For alternative meanings see proton (disambiguation). In physics, the proton (Greek proton = first) is a subatomic particle with an electric charge of one positive fundamental unit (1.602 × 10−19 coulomb) and a mass of 938.3 MeV/c2 (1.6726 × 10−27 kg), or about 1836 times the mass of an electron. The proton is observed to be stable, with a lower limit on its half-life of about 1035 years, although some theories predict that the proton may decay. The proton and neutron are both nucleons. The nucleus of the most common isotope (called protium) of the hydrogen atom is a single proton. The nuclei of other atoms are composed of protons and neutrons held together by the strong nuclear force. The number of protons in the nucleus determines the chemical properties of the atom and which chemical element it is. Protons are classified as baryons and are composed of two up quarks and one down quark, which are also held together by the strong nuclear force, mediated by gluons. The proton's antimatter equivalent is the antiproton, which has the same magnitude charge as the proton but the opposite sign. In chemistry and biochemistry, the term proton may refer to the hydrogen ion, H+. In this context, a proton donor is an acid and a proton acceptor a base (see acid-base reaction theories).

History

Ernest Rutherford is generally credited with the discovery of the proton. In 1918 Rutherford noticed that when alpha particles were shot into nitrogen gas, his scintillation detectors showed the signatures of hydrogen nuclei. Rutherford determined that the only place this hydrogen could have come from was the nitrogen, and therefore nitrogen must contain hydrogen nuclei. He thus suggested that the hydrogen nucleus, which was known to have an atomic number of 1, was an elementary particle. Prior to Rutherford, Eugene Goldstein had observed canal rays, which were composed of positively charged ions.

Technological applications

Protons can exist in spin states. This property is exploited by nuclear magnetic resonance spectroscopy. In NMR spectroscopy, a magnetic field is applied to a substance in order to detect the shielding around the protons in the nuclei of that substance, which is provided by the surrounding electron clouds. Scientists can use this information to then construct the molecular structure of the molecule under study.

Antiproton

The antiproton is the antiparticle of the proton. It was discovered in the year 1955 by Emilio Segre and Owen Chamberlain, for which they were awarded a 1959 Nobel Prize in Physics. CPT-symmetry puts strong constraints on the relative properties of particles and antiparticles and, therefore, is open to stringent tests. For example, the charges of the proton and antiproton must sum to exactly zero. This equality has been tested to one part in 10-8. The equality of their masses is also tested to better than one part in 10-8. By holding antiprotons in a Penning trap, the equality of the charge to mass ratio of the proton and the antiproton has been tested to 1 part in 9×10-11. The magnetic moment of the antiproton has been found with error of 8×10-3 nuclear Bohr magnetons, and is found to be equal and opposite to that of the proton.

See also


- particle physics
- subatomic particle
- quark model
- neutron
- proton-proton chain
- proton pump inhibitor
- proton therapy
- list of particles
- Fermion field

External links


- [http://pdg.lbl.gov/ Particle Data Group] Category:Nucleon ko:양성자 ms:Proton ja:陽子 simple:Proton th:โปรตอน

Speed of light

. The effect is due to electrons moving faster than the speed at which light moves in water.]] The speed of light in a vacuum is defined to be exactly 299,792,458 metres per second (or 1,079,252,848.8 km/h, which is approximately 186,282.397 miles per second, or 670,616,629.4 miles per hour). This value is denoted by the letter c, reputedly from the Latin celeritas, "speed", and also known as Einstein's constant. Note that this speed is a definition, not a measurement; in fact, the fundamental SI unit of distance, the metre, is defined in terms of the speed of light: it is the distance light travels in a vacuum in 1/299,792,458 of a second. The speed of light through a transparent medium (that is, not in vacuum) is less than c; the ratio of c to this speed is called the refractive index of the medium. "Speed of light" is sometimes abbreviated SOL.

Overview

According to standard modern physical theory, all electromagnetic radiation, including visible light, propagates (or moves) at a constant speed in a vacuum, commonly known as the speed of light, which is a physical constant denoted as c. This speed c is also the speed of propagation of gravity in the theory of general relativity. One consequence of the laws of electromagnetism (such as Maxwell's equations) is that the speed c of electromagnetic radiation does not depend on the velocity of the object emitting the radiation; thus for instance the light emitted from a rapidly moving light source would travel at the same speed as the light coming from a stationary light source (although the colour, frequency, energy, and momentum of the light will be shifted, which is called the relativistic Doppler effect). If one combines this observation with the principle of relativity, one concludes that all observers will measure the speed of light in vacuum as being the same, regardless of the reference frame of the observer or the velocity of the object emitting the light. Because of this, one can view c as a fundamental physical constant. This fact can then be used as a basis for the theory of special relativity. It is worth noting that it is the constant speed c, rather than light itself, which is fundamental to special relativity; thus if light is somehow manipulated to travel at more or less than c, this will not directly affect the theory of special relativity. Observers travelling at large velocities will find that distances and times are distorted ("dilated") in accordance with the Lorentz transforms; however, the transforms distort times and distances in such a way that the speed of light remains constant. A person travelling near the speed of light would also find that colours of lights ahead were blue shifted and of those behind were red shifted. If information could travel faster than c in one reference frame, causality would be violated: in some other reference frames, the information would be received before it had been sent, so the 'cause' could be observed after the 'effect'. Due to special relativity's time dilation, the ratio between an external observer's perceived time and the time perceived by an observer moving closer and closer to the speed of light approaches zero. If something could move faster than light, this ratio would not be a real number. Such a violation of causality has never been observed. real number and those that are not.]] To put it another way, information propagates to and from a point from regions defined by a light cone. The interval AB in the diagram to the right is 'time-like' (that is, there is a frame of reference in which event A and event B occur at the same location in space, separated only by their occurring at different times, and if A precedes B in that frame then A precedes B in all frames: there is no frame of reference in which event A and event B occur simultaneously). Thus, it is hypothetically possible for matter (or information) to travel from A to B, so there can be a causal relationship (with A the 'cause' and B the 'effect'). On the other hand, the interval AC in the diagram to the right is 'space-like' (that is, there is a frame of reference in which event A and event C occur simultaneously, separated only in space; see simultaneity). However, there are also frames in which A precedes C (as shown) or in which C precedes A. Barring some way of travelling faster than light, it is not possible for any matter (or information) to travel from A to C or from C to A. Thus there is no causal connection between A and C. According to the currently prevailing definition, adopted in 1983, the speed of light is exactly 299,792,458 metres per second (approximately 3 × 108 metres per second, or about thirty centimetres (one foot) per nanosecond). The value of c defines the permittivity of free space (\epsilon_0) in SI units as: : \varepsilon_0 = 10^/4\pi c^2 \quad \mathrm The permeability of free space (\mu_0) is not dependent on c and is defined in SI units as: : \mu_0 = 4\,\pi\, 10^ \quad \mathrm. These constants appear in Maxwell's equations, which describe electromagnetism, and are related by: :c= \frac Astronomical distances are sometimes measured in light years (the distance that light would travel in one year, roughly 9.46 × 1012 kilometres or about 5.88 × 1012 miles) especially in popularised texts.

Communications

The speed of light is of relevance to communications. For example, given that the equatorial circumference of the Earth is 40,075 km and c, the theoretical shortest amount of time for a piece of information to travel half the globe is 0.067 second. The actual transit time is longer, in part because the speed of light is slower by about 30% in an optical fibre and straight lines rarely occur in global communications situations, but also because delays are created when the signal passes through an electronic switch or signal regenerator. A typical time as of 2004 for an Australia or Japan to US computer-to-computer ping is 0.18 second. The speed of light additionally affects wireless communications design. The finite speed of light became quite apparent to everybody following the communication of Houston ground control and Neil Armstrong when he became the first man to set foot on the Moon: For every question, Houston had to wait nearly 3 seconds for the answer to arrive, and would have to do so even if the astronauts replied immediately. (See animation.) Similarly, instantaneous remote control of an interplanetary spacecraft is impossible, in the sense that the time it takes, for example, for the earth-based controllers to become aware of a problem, plus the time it takes for the spacecraft to receive their response, can be some hours. The speed of light can also be of concern on short distances. In supercomputers, the speed of light imposes a limit on how quickly data can be sent between processors. If a processor operates at 1 GHz, a signal can only travel a maximum of 300 mm in a single cycle. Processors must therefore be placed close to each other to minimise communication latencies. If clock frequencies continue to increase, the speed of light will eventually become a limiting factor for the internal design of single chips.

Physics

Constant velocity from all reference frames

It is important to realise that the speed of light is not a "speed limit" in the conventional sense. An observer chasing a beam of light will measure it moving away from him at the same speed as a stationary observer. This leads to some unusual consequences for velocities. Most individuals are accustomed to the addition rule of velocities: if two cars approach each other from opposite directions, each travelling at a speed of 50 kilometres per hour (31 miles per hour), one expects that each car will perceive the other as approaching at a combined speed of 50 + 50 = 100 km/h (62 mph) to a very high degree of accuracy. At velocities at or approaching the speed of light, however, it becomes clear from experimental results that this rule does not apply. Two spaceships approaching each other, each travelling at 90% the speed of light relative to some third observer between them, do not perceive each other as approaching at 90% + 90% = 180% the speed of light; instead they each perceive the other as approaching at slightly less than 99.5% the speed of light. This last result is given by the Einstein velocity addition formula: : u = \,\! where v and w are the speeds of the spaceships as observed by the third observer, and u is the speed of either space ship as observed by the other. Contrary to one's usual intuitions, regardless of the speed at which one observer is moving relative to another observer, both will measure the speed of an incoming light beam as the same constant value, the speed of light. The above equation was derived by Albert Einstein from his theory of special relativity, which takes the principle of relativity as a main premise. This principle (originally proposed by Galileo Galilei) requires physical laws to act in the same way in all reference frames. As Maxwell's equations directly give a speed of light, it should be the same for every observer—a consequence which sounded obviously wrong to the 19th century physicists, who assumed that the speed of light given by Maxwell's theory is valid relative to the luminiferous aether. But the Michelson-Morley experiment, arguably the most famous and useful failed experiment in the history of physics, could not find this aether, suggesting instead that the speed of light is constant in all frames of reference. Although it is uncertain whether Einstein knew the results of the Michelson-Morley experiment, he took the speed of light being constant as a given fact, understood it as reaffirming Galilei's principle of relativity, and deduced the consequences, now known as the theory of special relativity which includes the counter-intuitive addition formula above.

Interaction with transparent materials

special relativity, as demonstrated by this prism (in the case of a prism splitting white light into a spectrum of colours, the refraction is known as dispersion).]] In passing through materials, light is slowed to less than c by the ratio called the refractive index of the material. The speed of light in air is only slightly less than c. Denser media, such as water and glass, can slow light much more, to fractions such as 3/4 and 2/3 of c. This reduction in speed is also responsible for bending of light at an interface between two materials with different indices, a phenomenon known as refraction. Since the speed of light in a material depends on the refractive index, and the refractive index depends on the frequency of the light, light at different frequencies travels at different speeds through the same material. This can cause distortion of electromagnetic waves that consist of multiple frequencies, called dispersion. Note that the speed of light referred to is the observed or measured speed in some medium and not the true speed of light (as observed in vacuum). On the microscopic scale, considering electromagnetic radiation to be like a particle, refraction is caused by continual absorption and re-emission of the photons that compose the light by the atoms or molecules through which it is passing. In some sense, the light itself travels only through the vacuum existing between these atoms, and is impeded by the atoms. The process of absorption and re-emission itself takes time thereby creating the impression that the light itself has undergone delay (i.e. loss of speed) between entry and exit from the medium in question. It may be noted, that once the light has emerged from the medium it changes back to its original speed and this is without gaining any energy. This can mean only one thing - that the light's speed itself was never altered in the first place. Alternatively, considering electromagnetic radiation to be like a wave, the charges of each atom (primarily the electrons) interfere with the electric and magnetic fields of the radiation, slowing its progress.

"Faster-than-light" observations and experiments

It has long been known theoretically that it is possible for the group velocity of light to exceed c. One recent experiment made the group velocity of laser beams travel for extremely short distances through caesium atoms at 300 times c. However, it is not possible to use this technique to transfer information faster than c: the velocity of information transfer depends on the front velocity (the speed at which the first rise of a pulse above zero moves forward) and the product of the group velocity and the front velocity is equal to the square of the normal speed of light in the material. Exceeding the group velocity of light in this manner is comparable to exceeding the speed of sound by arranging people in a distantly spaced line, and asking them all to shout "I'm here!", one after another with short intervals, each one timing it by looking at their own wristwatch so they don't have to wait until they hear the previous person shouting. The speed of light may also appear to be exceeded in some phenomena involving evanescent waves, such as tunnelling. Experiments indicate that the phase velocity of evanescent waves may exceed c; however, it would appear that neither the group velocity nor the front velocity exceed c, so, again, it is not possible for information to be transmitted faster than c. In some interpretations of quantum mechanics, quantum effects may be transmitted at speeds greater than c (indeed, action at a distance has long been perceived as a problem with quantum mechanics: see EPR paradox). For example, the quantum states of two particles can be entangled, so the state of one particle fixes the state of the other particle (say, one must have spin +½ and the other must have spin −½). Until the particles are observed, they exist in a superposition of two quantum states, (+½, −½) and (−½, +½). If the particles are separated and one of them is observed to determine its quantum state then the quantum state of the second particle is determined automatically. If, as in some interpretations of quantum mechanics, one presumes that the information about the quantum state is local to one particle, then one must conclude that second particle takes up its quantum state instantaneously, as soon as the first observation is carried out. However, it is impossible to control which quantum state the first particle will take on when it is observed, so no information can be transmitted in this manner. The laws of physics also appear to prevent information from being transferred through more clever ways and this has led to the formulation of rules such as the no-cloning theorem. So-called superluminal motion is also seen in certain astronomical objects, such as the jets of radio galaxies and quasars. However, these jets are not actually moving at speeds in excess of the speed of light: the apparent superluminal motion is a projection effect caused by objects moving near the speed of light and at a small angle to the line of sight. Although it may sound paradoxical, it is possible for shock waves to be formed with electromagnetic radiation. As a charged particle travels through an insulating medium, it disrupts the local electromagnetic field in the medium. Electrons in the atoms of the medium will be displaced and polarised by the passing field of the charged particle, and photons are emitted as the electrons in the medium restore themselves to equilibrium after the disruption has passed. (In a conductor, the disruption can be restored without emitting a photon.) In normal circumstances, these photons destructively interfere with each other and no radiation is detected. However, if the disruption travels faster than the photons themselves travel, the photons constructively interfere and intensify the observed radiation. The result (analogous to a sonic boom) is known as Cherenkov radiation. The ability to communicate or travel faster-than-light is a popular topic in science fiction. Particles that travel faster than light, dubbed tachyons, have been proposed by particle physicists but have yet to be observed. Some physicists, notably João Magueijo and John Moffat, have proposed that in the past light travelled much faster than the current speed of light. This theory is called variable speed of light (VSL) and its supporters claim that it has the ability to explain many cosmological puzzles better than its rival, the inflation model of the universe. However, it has yet to gain wide acceptance.

Light-slowing experiments

universe, are due to the slower speed of light in a medium (water, in this case).]] In a sense, any light travelling through a medium other than a vacuum travels below c as a result of refraction. However, certain materials have an exceptionally high refractive index: in particular, the optical density of a Bose-Einstein condensate can be very high. In 1999, a team of scientists led by Lene Hau were able to slow the speed of a light beam to about 17 metres per second, and, in 2001, they were able to momentarily stop a beam. In 2003, Mikhail Lukin, with scientists at Harvard University and the Lebedev Institute in Moscow, succeeded in completely halting light by directing it into a mass of hot rubidium gas, the atoms of which, in Lukin's words, behaved "like tiny mirrors", due to an interference pattern in two "control" beams.

History

Until relatively recent times, the speed of light was largely a matter of conjecture. Empedocles maintained that light was something in motion, and therefore there had to be some time elapsed in travelling. Aristotle said that, on the contrary, "light is due to the presence of something, but it is not a movement". Furthermore, if light had a finite speed, it would have to be very great; Aristotle asserted "the strain upon our powers of belief is too great" to believe this. One of the ancient theories of vision is that light is emitted from the eye, instead of being reflected into the eye from another source. On this theory, Heron of Alexandria advanced the argument that the speed of light must be infinite, since distant objects such as stars appear immediately when one opens one's eyes.

Medieval and early modern theories

The Islamic philosophers Avicenna and Alhazen believed that light has a finite speed, although most philosophers agreed with Aristotle on this point. The Aryan school of philosophy in ancient India also held the speed of light to be finite. The 14th century philosopher Sayana wrote the following comment on verse 1.50 of the Rig Veda: :"Thus it is remembered: [O Sun] you who traverse 2202 yojanas in half a nimesa." According to some, this refers to the speed of light. It is not known exactly how long a yojana and a nimesa is, but this value is possibly accurate to within 1% (Kak, 1998), though by adopting other possible values of these units the accuracy of the statement can be reduced to a factor of 4. Johannes Kepler believed that the speed of light is infinite since empty space presents no obstacle to it. Francis Bacon argued that the speed of light is not necessarily infinite, since something can travel too fast to be perceived. René Descartes argued that if the speed of light were finite, the Sun, Earth, and Moon would be noticeably out of alignment during a lunar eclipse. Since such misalignment had not been observed, Descartes concluded the speed of light is infinite. In fact, Descartes was convinced that if the speed of light were finite, his whole system of philosophy would be demolished.

Measurement of the speed of light

Isaac Beeckman, a friend of Descartes, proposed an experiment (1629) in which one would observe the flash of a cannon reflecting off a mirror about one mile away. Galileo proposed an experiment (1638), with an apparent claim to have performed it some years earlier, to measure the speed of light by observing the delay between uncovering a lantern and its perception some distance away. Descartes criticised this experiment as superfluous, in that the observation of eclipses, which had more power to detect a finite speed, gave a negative result. This experiment was carried out by the Accademia del Cimento of Florence in 1667, with the lanterns separated by about one mile. No delay was observed. Robert Hooke explained the negative results as Galileo had: by pointing out that such observations did not establish the infinite speed of light, but only that the speed must be very great. The first quantitative estimate of the speed of light was made in 1676 by Ole Rømer, who was studying the motions of Jupiter's satellite Io with a telescope. It is possible to time the revolution of Io because it is entering/exiting Jupiter's shadow at regular intervals. Rømer observed that Io revolved around Jupiter once every 42.5 hours when Earth was closest to Jupiter. He also observed that, as Earth and Jupiter moved apart, Io's exit from the shadow would begin progressively later than predicted. It was clear that these exit "signals" took longer to reach Earth, as Earth and Jupiter moved further apart, as a result of the extra time it took for light to cross the extra distance between the planets, which had accumulated in the interval between one signal and the next. Similarly, about half a year later, Io's entries into the shadow happened more frequently, as Earth and Jupiter were now drawing closer together. On the basis of his observations, Rømer estimated that it would take light 22 minutes to cross the diameter of the orbit of the Earth (that is, twice the astronomical unit); the modern estimate is closer to 16 minutes and 40 seconds. Around the same time, the astronomical unit was estimated to be about 140 million kilometres. The astronomical unit and Rømer's time estimate were combined by Christiaan Huygens, who estimated the speed of light to be 1000 Earth diameters per minute. This is about 220,000 kilometres per second (136,000 miles per second), well below the currently accepted value, but still very much faster than any physical phenomenon then known. Isaac Newton also accepted the finite speed. In his book "Opticks" he, in fact, reports the more accurate value of 16 minutes per diameter, which it seems he inferred for himself (whether from Rømer's data, or otherwise, is not known). The same effect was subsequently observed by Rømer for a "spot" rotating with the surface of Jupiter. And later observations also showed the effect with the three other Galilean moons, where it was more difficult to observe, thus laying to rest some further objections that had been raised. Even if, by these observations, the finite speed of light may not have been established to everyone's satisfaction (notably Jean-Dominique Cassini's), after the observations of James Bradley (1728), the hypothesis of infinite speed was considered discredited. Bradley deduced that starlight falling on the Earth should appear to come from a slight angle, which could be calculated by comparing the speed of the Earth in its orbit to the speed of light. This "aberration of light", as it is called, was observed to be about 1/200 of a degree. Bradley calculated the speed of light as about 185,000 miles per second (298,000 kilometres per second). This is only slightly less than the currently accepted value. The aberration effect has been studied extensively over the succeeding centuries, notably by Friedrich Georg Wilhelm Struve and Magnus Nyren. Magnus Nyren The first successful measurement of the speed of light using an earthbound apparatus was carried out by Hippolyte Fizeau in 1849. Fizeau's experiment was conceptually similar to those proposed by Beeckman and Galileo. A beam of light was directed at a mirror several thousand metres away. On the way from the source to the mirror, the beam passed through a rotating cog wheel. At a certain rate of rotation, the beam could pass through one gap on the way out and another on the way back. But at slightly higher or lower rates, the beam would strike a tooth and not pass through the wheel. Knowing the distance to the mirror, the number of teeth on the wheel, and the rate of rotation, the speed of light could be calculated. Fizeau reported the speed of light as 313,000 kilometres per second. Fizeau's method was later refined by Marie Alfred Cornu (1872) and Joseph Perrotin (1900). Leon Foucault improved on Fizeau's method by replacing the cogwheel with a rotating mirror. Foucault's estimate, published in 1862, was 298,000 kilometres per second. Foucault's method was also used by Simon Newcomb and Albert A. Michelson. Michelson began his lengthy career by replicating and improving on Foucault's method. In 1926, Michelson used rotating mirrors to measure the time it took light to make a round trip from Mount Wilson to Mount San Antonio in California. The precise measurements yielded a speed of 186,285 miles per second (299,796 kilometres per second).

Relativity

Due to the works of James Clerk Maxwell, it was known that the speed of electromagnetic radiation was a constant defined by the electromagnetic properties of the vacuum (permittivity and permeability). permeability, as used for the Michelson-Morley experiment.]] In 1887, the physicists Albert Michelson and Edward Morley performed the influential Michelson-Morley experiment to measure the speed of light relative to the motion of the earth, the goal being to measure the velocity of the Earth through the "luminiferous aether", the medium that was then thought to be necessary for the transmission of light. As shown in the diagram of a Michelson interferometer, a half-silvered mirror was used to split a beam of monochromatic light into two beams travelling at right angles to one another. After leaving the splitter, each beam was reflected back and forth between mirrors several times (the same number for each beam to give a long but equal path length; the actual Michelson-Morley experiment used more mirrors than shown) then recombined to produce a pattern of constructive and destructive interference. Any slight change in speed of light along each arm of the interferometer (due to the fact that the apparatus was moving with the Earth through the proposed "aether") would change the amount of time that the beam spent in transit, which would then be observed as a change in the pattern of interference. In the event, the experiment gave a null result. Ernst Mach was among the first physicists to suggest that the experiment actually amounted to a disproof of the aether theory. Developments in theoretical physics had already begun to provide an alternate theory, Fitzgerald-Lorentz contraction, which explained the null result of the experiment. It is uncertain whether Albert Einstein knew the results of the Michelson-Morley experiment, but the null result of the experiment greatly assisted the acceptance of his theory of relativity. Einstein's theory did not require an aether and was entirely consistent with the null result of the experiment: the aether did not exist and the speed of light was the same in each direction. The constant speed of light is one of the fundamental Postulates (together with causality and the equivalence of inertial frames) of special relativity.

See also


- Fizeau-Foucault apparatus
- variable speed of light

References

Historical references


- Ole Rømer. "Démonstration touchant le mouvement de la lumière", Journal des Sçavans, 7 Décembre 1676, pp. 223-236. Translated as "A Demonstration concerning the Motion of Light", Philosophical Transactions of the Royal Society no. 136, pp. 893-894; June 25, 1677. (Rømer's 1676 paper, in English and French, as bitmap images: [http://dbhs.wvusd.k12.ca.us/webdocs/Chem-History/Roemer-1677/Roemer-1677.html], and in French as plain text: [http://astro.campus.ecp.fr/histoire/roemer.html])
- Edmund Halley. "Monsieur Cassini, his New and Exact Tables for the Eclipses of the First Satellite of Jupiter, reduced to the Julian Stile and Meridian of London", Philosophical Transactions XVIII, No. 214, pp 237–256, Nov.–Dec., 1694.
- H.L. Fizeau. "Sur une experience relative a la vitesse de propogation de la lumiere", Comptes Rendus 29, 90–92, 132, 1849.
- J.L. Foucault. "Determination experimentale de la vitesse de la lumiere: parallaxe du Soleil", Comptes Rendus 55, 501–503, 792–796, 1862.
- A.A. Michelson. "Experimental Determination of the Velocity of Light", Proceedings of the American Association for the Advancement of Science 27, 71–77, 1878. ([http://www.gutenberg.org/etext/11753 Project Gutenberg Etext])
- Simon Newcomb. "The Velocity of Light", Nature, pp 29–32, May 13, 1886.
- Joseph Perrotin. "Sur la vitesse de la lumiere", Comptes Rendus 131, 731–734, 1900.
- A.A. Michelson, F.G. Pease, and F. Pearson. "Measurement Of The Velocity Of Light In A Partial Vacuum", Astrophysical Journal 82, 26–61, 1935.

Modern references


- Léon Brillouin. Wave propagation and group velocity. Academic Press Inc., 1960.
- John David Jackson. Classical electrodynamics. John Wiley & Sons, 2nd edition, 1975; 3rd edition, 1998. ISBN 047130932X
- Subhash Kak. The Speed of Light and Purāṇic Cosmology. In T.R.N. Rao and S. Kak, Computing Science in Ancient India, pages 80–90. USL Press, Lafayette, 1998. Available as [http://uk.arxiv.org/abs/physics/9804020 e-print physics/9804020] on the arXiv.
- R.J. MacKay and R.W. Oldford. "Scientific Method, Statistical Method and the Speed of Light", Statistical Science 15(3):254–278, 2000. (Also available on line: [http://www.stats.uwaterloo.ca/~rwoldfor/papers/sci-method/paperrev])

External links


- [http://physics.nist.gov/cgi-bin/cuu/Value?c speed of light in vacuum] (at NIST)
- [http://www.ldolphin.org/chistory.html A Brief History of c]
- [http://www.itl.nist.gov/div898/bayesian/datagall/michelso.htm Data Gallery: Michelson Speed of Light (Univariate Location Estimation)] (download data gathered by A.A. Michelson)
- [http://physicsweb.org/article/news/7/12/5 Switching light on and off] (news article on stopping light)
- [http://news.bbc.co.uk/hi/english/sci/tech/newsid_841000/841690.stm Beam smashes light barrier] (news article on group velocity experiment)
- [http://www.netspace.net.au/~gregegan/APPLETS/20/20.html Subluminal] (Java applet demonstrating group velocity information limits)
- [http://www.ertin.com/sloan_on_speed_of_light.html Light discussion on adding velocities] Category:Electromagnetic radiation Category:Units of velocity Category:Special relativity als:Lichtgeschwindigkeit ko:빛의 속도 ms:Kelajuan cahaya ja:光速度 simple:Speed of light th:อัตราเร็วของแสง

Electromagnetic radiation

Electromagnetic radiation is a propagating wave in space with electric and magnetic components. These components oscillate at right angles to each other and to the direction of propagation. The term electromagnetic radiation is also used as a synonym for electromagnetic waves in general, even if they are not radiating or travelling in free space. This sense includes, for example, light travelling through an optical fiber, or electrical energy travelling within a coaxial cable. Electromagnetic (EM) radiation carries energy and momentum which may be imparted when it interacts with matter.

Physics

Theory

Electromagnetic waves of much lower frequency than visible light were predicted by Maxwell's equations and subsequently discovered by Heinrich Hertz. Maxwell derived a wave form of the electric and magnetic equations which made explicit the wave nature of the electric and magnetic fields. These equations displayed the symmetry of the fields. According to the theory, a time-varying electric field generates a magnetic field and vice versa. Thus, an oscillating electric field creates an oscillating magnetic field, which in turn creates an oscillating electric field, and so on. By this means an EM wave is produced which propagates through space.

Properties

Electric and magnetic fields exhibit the property of superposition. This means that the field due to a particular particle or time-varying electric or magnetic field adds to the fields due to other causes. (As magnetic and electric fields are vector fields, this is the vector addition of all the individual electric and magnetic field vectors.) As a result, EM radiation is influenced by various phenomena such as refraction and diffraction. For example, a travelling EM wave incident on a particular arrangement of atoms induces oscillation in the atoms and thus causes them to emit their own EM waves (called wavelets). These emissions interfere with the impinging wave and alter its form. In refraction, a wave moving from one medium to another of a different density changes its speed and direction when it enters the new medium. The ratio of the refractive indices of the media determines the extent of refraction. Refraction is the mechanism by which light disperses into a spectrum when it is shone through a prism. The physics of electromagnetic radiation is electrodynamics, a subfield of electromagnetism. EM radiation exhibits both wave properties and particle properties at the same time (see wave-particle duality). These characteristics are mutually exclusive and appear separately in different circumstances: the wave characteristics appear when EM radation is measured over relatively larger timescales and over larger distances, and the particle characteristics are evident when measuring smaller distances and timescales. EM radiation's behaviours as a wave and as a stream of particles have been confirmed by a large number of experiments.

Wave model

An important aspect of the wave nature of light is frequency. The frequency of a wave is its rate of oscillation and is measured in hertz, the SI unit of frequency, equal to one oscillation per second. Light usually comprises a spectrum of frequencies which sum to form the resultant wave. In addition, frequency affects properties like refraction, in which different frequencies undergo a different level of refraction. A wave has troughs and crests. The wavelength is the distance from crest to crest. Waves in the electromagnetic spectrum vary in size from very long radio waves the size of buildings, to very short gamma-rays smaller than the size of the nucleus of an atom. Frequency has an inverse relationship to the concept of wavelength. When waves travel from one medium to another, their frequency remains exactly the same - only their wavelength and/or speed changes. Waves can also be described by their radiant energy. Interference is the superposition of two or more waves resulting in a new wave pattern. The way that these coincide causes different types of interference.

Particle model

In the particle model of EM radiation, EM radiation is quantized as particles called photons. Quantisation of light represents the discrete packets of energy which constitute the radiation. The frequency of the radiation determines the magnitude of the energy of the particles. Moreover, these particles are emitted and absorbed by charged particles, so photons act as transporters of energy. A photon absorbed by an atom excites an electron and elevates it to a higher energy level. If the energy is great enough, the electron is liberated from the atom in a process called ionization. Conversely, an electron which descends to a lower energy level in an atom emits a photon of light equal to the energy difference. The energy levels of electrons in atoms are discrete. Therefore, each element has its own characteristic frequencies. Together these effects explain the absorption spectra of light. The dark bands in the spectrum are due to the atoms in the intervening medium which absorb different frequencies of the light. The composition of the medium through which the light travels determines the nature of the absorption spectrum. For instance, in a distant star, dark bands in the light it emits are due to the atoms in the atmosphere of the star. These bands correspond to the allowed energy levels in the atoms. A similar phenomenon occurs for emission. As the electrons descend to lower energy levels, a spectrum which represents the jumps between the energy levels of the electrons is exhibited. This is manifested in the emission spectrum of nebulae.

Speed of propagation

Any electric charge which accelerates, or any changing magnetic field, produces electromagnetic radiation. Electromagnetic information about the charge travels at the speed of light. Accurate treatment thus incorporates a concept known as retarded time (as opposed to advanced time, which is unphysical in light of causality), which adds to the expressions for the electrodynamic electric field and magnetic field. These extra terms are responsible for electromagnetic radiation. When any wire (or other conducting object such as an antenna) conducts alternating current, electromagnetic radiation is propagated at the same frequency as the electric current. Depending on the circumstances, it may behave as a wave or as particles. As a wave, it is characterized by a velocity (the speed of light), wavelength, and frequency. When considered as particles, they are known as photons, and each has an energy related to the frequency of the wave given by Planck's relation E = hν, where E is the energy of the photon, h = 6.626 × 10-34 J·s is Planck's constant, and ν is the frequency of the wave. One rule is always obeyed regardless of the circumstances. EM radiation in a vacuum always travels at the speed of light, relative to the observer, regardless of the observer's velocity. (This observation led to Albert Einstein's development of the theory of special relativity.)

Electromagnetic spectrum

Generally, EM radiation is classified by wavelength into electrical energy, radio, microwave, infrared, the visible region we perceive as light, ultraviolet, X-rays and gamma rays. The behavior of EM radiation depends on its wavelength. Higher frequencies have shorter wavelengths, and lower frequencies have longer wavelengths. When EM radiation interacts with single atoms and molecules, its behavior depends on the amount of energy per quantum it carries. Spectroscopy can detect a much wider region of the EM spectrum than the visible range of 400 nm to 700 nm. A common laboratory spectroscope can detect wavelengths from 2 nm to 2500 nm. More in-depth information about the physical properties of objects, gases, or even stars can be obtained from this type of device. It is widely used in astrophysics. For example, many hydrogen atoms emit radio waves which have a wavelength of 21.12 cm.

Light

EM radiation with a wavelength between 400 nm and 700 nm is detected by the human eye and perceived as visible light. If radiation having a frequency in the visible region of the EM spectrum shines on an object, say, a bowl of fruit, this results in our visual perception of identifying information from the scene. Our brain's visual system processes the multitude of reflected frequencies into different shades and hues, and through this not-entirely-understood "psychophysical phenomenon," most humans perceive a bowl of fruit. In the vast majority of cases, however, the information carried by light is not directly apprehensible by human senses. Natural sources produce EM radiation across the spectrum; so, too, can human technology manipulate a broad range of wavelengths. Optical fiber transmits light which, although not suitable for direct viewing, can carry data. Those data can be translated into sound or an image. The coded form of such data is similar to that used with radio waves.

Radio waves

Radio waves carry information by varying amplitude and by varying frequency within a frequency band. When EM radiation impinges upon a conductor, it couples to the conductor, travels along it, and induces an electric current on the surface of that conductor by exciting the electrons of the conducting material. This effect (the skin effect) is used in antennas. EM radiation may also cause certain molecules to absorb energy and thus to heat up; this is exploited in microwave ovens.

Derivation

Electromagnetic waves as a general phenomenon were predicted by the classical laws of electricity and magnetism, known as Maxwell's equations. If you inspect Maxwell's equations without sources (charges or currents) then you will find that, along with the possibility of nothing happening, the theory will also admit nontrivial solutions of changing electric and magnetic fields. (For symbol definitions see magnetic field.) :\nabla \cdot \mathbf = 0 :\nabla \times \mathbf = -\frac \mathbf :\nabla \cdot \mathbf = 0 :\nabla \times \mathbf = \mu_0 \epsilon_0 \frac \mathbf \mathbf=\mathbf=\mathbf is a solution, but there might be other solutions as well. Let us employ a useful identity from vector calculus. :\nabla \times \left( \nabla \times \mathbf \right) = \nabla \left( \nabla \cdot \mathbf \right) - \nabla^2 \mathbf Where \mathbf can be any vector function. Taking the curl of the curl equations and applying the identity, we get the following. :\nabla^2 \mathbf = \mu_0 \epsilon_0 \frac \mathbf :\nabla^2 \mathbf = \mu_0 \epsilon_0 \frac \mathbf These types of equations are identified as linear wave equations with wave speed \frac. Amazingly, this speed happens to be exactly the speed of light! Maxwell's equations have unified the permittivity of free space \epsilon_0, the permeability of free space \mu_0, and the speed of light itself: c = \frac. Before this derivation it was not known that there was such a strong relationship between light and electricity and magnetism. But these are only two equations and we started with four, so there is still more information pertaining to these waves hidden within Maxwell's equations. Let's consider a generic vector wave for the electric field. :\mathbf = \mathbf_0 f\left( \hat \cdot \mathbf - c t \right) Here \mathbf_0 is the constant amplitude, f is any second differentiable function, \hat is a unit vector in the direction of propagation, and \hat is a position vector. We observe that f\left( \hat \cdot \mathbf - c t \right) is a generic solution to the wave equation. In other words :\nabla^2 f\left( \hat \cdot \mathbf - c t \right) = \frac \frac f\left( \hat \cdot \mathbf - c t \right), for a generic wave traveling in the \hat direction. The proof of this is trivial. This form will satisfy the wave equation, but will it satisfy all of Maxwell's equations, and with what corresponding magnetic field? :\nabla \cdot \mathbf = \hat \cdot \mathbf_0 f'\left( \hat \cdot \mathbf - c t \right) = 0 :\mathbf \cdot \hat = 0 The first of Maxell's equations implies that electric field is orthogonal to the direction the wave propagates. :\nabla \times \mathbf = \hat \times \mathbf_0 f'\left( \hat \cdot \mathbf - c t \right) = -\frac \mathbf :\mathbf = \frac \hat \times \mathbf The second of Maxwell's equations yields the magnetic field. The remaining equations will be satisfied by this choice of \mathbf,\mathbf. Not only are the electric and magnetic field waves traveling at the speed of light, but they have a special restricted orientation and proportional magnitudes, \mathbf_0 = c \mathbf_0. The electric field, magnetic field, and direction of wave propagation are all orthogonal and the wave propagates in the same direction as \mathbf \times \mathbf. Visualizing yourself as an electromagnetic wave traveling forward, the electric field might be oscillating up and down, while the magnetic field oscillates right and left; but you can rotate this picture around with the electric field oscillating right and left and the magnetic field oscillating down and up. This is a different solution that is traveling in the same direction. This arbitrariness in the orientation, with respect to propagation direction, is known as polarization.

See also


- Electromagnetic wave equation
- Electromagnetic spectrum
- Electromagnetic radiation hazards
- Radiant energy
- Light
- Electromagnetic pulse
- Control of electromagnetic radiation
- Klystron

References


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External links

; General
- [http://www.sengpielaudio.com/calculator-wavelength.htm Conversion of frequency to wavelength and back - electromagnetic, radio and sound waves]
- [http://www.scienceofspectroscopy.info The Science of Spectroscopy - a learning tool for spectroscopy] ; Patents
- Greenleaf Whittier Pickard - - Intelligence intercommunication by magnetic wave component ko:전자기파 ja:電磁波

Electromagnetic spectrum

s
SX = Soft X-Rays
EUV = Extreme ultraviolet
NUV = Near ultraviolet
Visible light
NIR = Near infrared
MIR = Moderate infrared
FIR = Far infrared

Radio waves:
EHF = Extremely high frequency (Microwaves)
SHF = Super high frequency (Microwaves)
UHF = Ultrahigh frequency
VHF = Very high frequency
HF = High frequency
MF = Medium frequency
LF = Low frequency
VLF = Very low frequency
VF = Voice frequency
ELF = Extremely low frequency]] The electromagnetic spectrum is the range of all possible electromagnetic radiation. Also, the "electromagnetic spectrum" (usually just spectrum) of an object is the range of electromagnetic radiation that it emits, reflects, or transmits. The electromagnetic spectrum, shown in the table, extends from frequencies used in the electric power grid (at the long-wavelength end) to gamma radiation (at the short-wavelength end), covering wavelengths from thousands of kilometres down to fractions of the size of an atom, though in principle the spectrum is actually infinite. Electromagnetic energy at a particular wavelength λ (in vacuum) has an associated frequency ν and photon energy E. Thus, the electromagnetic spectrum may be expressed equally well in terms of any of these three quantities. They are related according to the equations: :\lambda = \frac \,\! and :E=h\nu \,\! where:
- c is the speed of light, 299792458 m/s (c \approx 3 \cdot 10^8 \ \mbox/\mbox = 300,000 \ \mbox/\mbox).
- h is Planck's constant, (h \approx 6.626069 \cdot 10^ \ \mbox \cdot \mbox \approx 4.13567 \ \mathrm \mbox/\mbox).

Spectra of objects

Nearly all objects in the universe emit, reflect and/or transmit some light. The distribution of this light along the electromagnetic spectrum (called the spectrum of the object) is determined by the object's composition. Several types of spectra can be distinguished depending upon the nature of the radiation coming from an object:
- If the spectrum is composed primarily of thermal radiation emitted by the object itself, an emission spectrum occurs.
  - Some bodies emit light more or less according to the blackbody spectrum.
- If the spectrum is composed of background light, parts of which the object transmits and parts of which it absorbs, an absorption spectrum occurs. Electromagnetic spectroscopy is the branch of physics that deals with the characterization of matter by its spectra.

Classification systems

While the classification scheme is generally accurate, in reality there is often some overlap between neighboring types of electromagnetic energy. For example, SLF radio waves at 60 Hz may be received and studied by astronomers, or may be ducted along wires as electric power. Also, some low-energy gamma rays actually have a longer wavelength than some high-energy X-rays. This is possible because "gamma ray" is the name given to the photons generated from nuclear decay or other nuclear and subnuclear processes, whereas X-rays on the other hand are generated by electronic transitions involving highly energetic inner electrons. Therefore the distinction between gamma ray and X-ray is related to the radiation source rather than the radiation wavelength. Generally, nuclear transitions are much more energetic than electronic transitions, so usually, gamma-rays are more energetic than X-rays. However, there are a few low-energy nuclear transitions (e.g. the 14.4 keV nuclear transition of Fe-57) that produce gamma rays that are less energetic than some of the higher energy X-rays. Use of the radio frequency spectrum is regulated by governments. This is called frequency allocation.

Electric energy

Electrical energy covers the low-frequency, long-wavelength end of the spectrum. The radiation is usually ducted along 2-wire and 3-wire transmission lines and sent to various devices besides antennas. At zero frequency the energy is emitted by batteries and DC power supplies, while at 50 Hz and 60 Hz it is produced by rotary magnetic generators and ducted through the international power grids. At frequencies between 20 Hz to 30 kHz the EM energy is translated to and from acoustic energy and is di