Question

What is critical angle? Explain it with a neat ray diagram.

Answer 1

When light leaves a medium of higher index $n_1$ and enters one of lower index $n_2$, the angle of refraction will be larger than the angle of incidence. Hence the outgoing ray will be bent towards the surface. As the angle of incidence is increased, the angle of refraction will eventually reach ${90}^{\circ }$. Beyond that angle of incidence, Snell's law requires $\sin r$ to be greater than unity. When that occurs, there is no mathematical solution for the outgoing angle, and hence there is no refracted ray at all. Instead, all of the light (instead of just some of it) is reflected from the surface. Such an angle is called critical angle.

It is obtained as $n_1\sin i_c=n_2$

$⟹i_c=si{n}^{-1}\frac{n_2}{n_1}$