Explain refractive index, critical angle.

Critical angle : When a light ray travelling from optically denser medium(high refractive index) to optically rarer medium(low refractive index) it undergoes refraction. Due to this, for a given angle of incidence at interface of two medium, light ray deviates its path in rarer medium away from normal. If we keep on increase the angle of incidence, at particular angle of incidence the light ray passes over the surface of denser medium giving an angle of refraction that equals 90º. This angle of incidence for which we get angle of refraction at rarer medium equals 90º is known as critical angle. If the angle of incidence exceeds this critical angle, there will not be any refraction and total internal reflection is taking place.
Refractive Index: A ray of light changes its direction when it enters from one medium to another medium. This happens because speed of light is different in different media. For example; the speed of light is 3 x 10⁸ m/s (2.99x10⁸ m/s) in vacuum and it is 2.98 x 10⁸ m/s in air. Refractive Index is the extent of change of direction of light in a given pair of media. The refractive index is a relative value of speed of light in the given pair of media. Thus, to calculate the refractive Index the speed of light in two media is taken.