incidence of 80 ı, Snell’s law (Eq. 46.2) gives the sine of the angle of refraction to be 1.31, so the angle
of refraction is undefined. No refraction occurs in this case: instead, the light will reflect off of the water-
air interface (following the law of reflection), and go back into the water. This phenomenon is calledtotal
internal reflection.
The critical angle for total internal reflection is given by
sincDn 2
n 1; (46.3)
wherecis the critical angle, the light is incident from medium 1, andn 1 andn 2 are the indices of refraction
of the two media (n 1 >n 2 ). For example, the critical angle for total internal reflection for an air-water
interface isc Dsin^1 .1:00=1:33/D 48:75ı; this means that any light ray in water headed toward an
interface with air will be reflected back into the water if its angle of incidence is greater than48:75ı. If its
angle of incidence is less than this critical angle, the ray will be refracted out into the air.