# Question #3474d

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.

#### Explanation: 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 $= {90}^{\circ}$, 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 ${42}^{\circ}$ and for diamond, it is $24 - {25}^{\circ}$.

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