Critical angle :
When light ray travelling from denser medium to rarer medium, then the angle of incidence for which angle of refraction in air is 90° is called critical angle.
C = sin-1(1/µ)
Total internal reflection : When a light ray travels from denser to rarer medium, the angle of incidence is greater than the critical angle, then it reflects into the same medium is called total internal reflection.
Explanation : Consider an object in the denser medium. A ray OA incident on XY bends away from the normal. As the angle of incidence is increased, the angle of refraction goes on increasing.
For certain angle of incidence, the refracted ray parallel to XY surface (r = 90°)
When the angle of incidence is further increased, the ray is not refracted but is totally reflected back in the denser medium. This phenomenon is called total internal reflection.
