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Refracció

Refracció

La refracció és el procés pel qual, quan una ona incideix sobre la superfície de separació entre dos medis, una part de la seva energia es trasmet al segon medi canviant la seva direcció de propagació. La relació entre la direcció del raig incident i la del raig refractat es pot obtenir mitjançant la llei d'Snell: si n_1 i n_2 són els índexs de refracció dels dos medis i si \theta_1 i \theta_2 són els angles que formen el raig incident i el refractat respecte a una línia perpendicular a la superfície, es compleix que
n_1 \sin\theta_1 = n_2 \sin\theta_2 \,
Aquest resultat fou descobert per Willebrod Snell el 1621 i, per aquesta raó es coneix com a llei d'Snell; des d'un punt de vista més fonamental, es pot deduir a partir del principi de Fermat. A partir de l'equació anterior, l'angle de desviació (\theta_2) es pot obtenir com:
\theta_2 = \arcsin \left( \frac \sin\theta_1 \right)
i amb aquesta expressió resulta interessant estudiar els dos casos possibles: n_1 < n_2 (per exemple llum que passa d'aire a aigua) i n_1 > n_2 (per exemple llum que passa de vidre a aigua). En el primer cas l'angle de desviació \theta_2 sempre serà més petit que l'angle d'incidència. En el segon cas l'angle de desviació és sempre superior al d'incidència; en aquest cas, quan l'angle de refracció arriba a 90º es produeix el fenomen de la reflexió total interna, de manera que deixa d'haver-hi refracció i tota la llum és reflectida. L'angle d'incidència per al qual l'angle de refracció és de 90º s'anomena angle crític \theta_c i es pot trobar fàcilment el seu valor tenint en compte que sin 90 = 1, de manera que
\theta_c = \arcsin \left( \frac \right)
Per a una interfície vidre-aigua, per exemple, l'angle crític és de 48,7º, de manera que qualsevol raig que passi d'aigua a aire i el seu angle d'incidència superi aquest valor, serà reflectit totalment. Categoria:Òptica ja:屈折

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The northern terminus of Route 132 is at Route 6A in West Barnstable.
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