Question

The speed of the light in a medium is three times less than its speed in the vacuum. Find the critical angle when light travels from this medium into the glass.

Answer 1

The expression for the critical angle is:

$$\theta_c = \arcsin(v_{medium} /v_{glass})$$

where $v_{medium} = c/3$ and $v_{glass} = c/1.5$ are speed of the light in a medium and in the glass respectively. Thus:

$$\theta_c = \arcsin(v_{medium} /v_{glass}) =\arcsin(\dfrac{c/3}{c/1.5}) = 30\degree$$

Answer. The critical angle is 30 degrees.