Critical angle: (2M) It is smallest possible angle of incidence at which light rays are totally reflected at an interface between substances of different refractive indices. Critical angle ($Ø_c$) = $\sin^{-1}( \frac{n_2}{n_1})$

Snell’s law:

1. How a light ray reacts when it meets the interface of two transmissive materials that have different indices of refraction can be explained with Snell’s law.
2. A refractive index model for Snell’s law is shown in figure below.

1. At the interface of medium 1 and medium 2, the incident ray may be refracted toward the normal or away from it, depending on whether 1 is greater than or less than 2. Hence angle of refraction can be greater or smaller than the angle of incidence, depending on the refractive indices of the two materials.

2. The relationship at the interface is known as Snell’s law and is given by

   $ \eta_1 sin \phi_1$=$ \eta_1 sin \phi_2$

   or equivalently,

   $ \eta_1 cos \phi_1$=$ \eta_1 cos \phi_2$

   where the angles are defined in the figure above.