Question #3474d

1 Answer
Jan 23, 2017

The angle of incidence for an incident ray in an optically denser medium for which the angle of refraction is 90 degrees is called critical angle.


BEFORE talking about the critical angle, let's understand total internal reflection.

CASE 1. A ray of light originates from water at an angle #theta_1#. Since it is going from a denser medium to a less dense medium, it bends away from the normal.

CASE 2. A ray originates at an angle #theta_2#, where #theta_2 >theta_1#, and refracts. Since the angle of incidence increases, the angle of refraction also increases.

When #theta_2# achieves a specific value such that the angle of refraction #= 90^@#, then that angle of incidence is called critical angle.

CASE 3. When the angle of incidence increases to such a value that it is greater than the critical angle, the light reflects into the same denser medium. Hence reflection with refraction occurs.

For water, the critical angle is #42^@# and for diamond, it is #24-25^@#.

Remember that for total internal reflection, the angle of incidence should be greater than the critical angle.