Total internal reflection is an optical phenomenon that occurs when a ray of light strikes a medium boundary at an angle larger than a particular critical angle between two media and that perticular wavelenght of light. If the refractive index is lower on the other side of the boundary, no light can pass through and all of the light is reflected.
When light crosses a boundary between materials with different refractive indices, the light beam will be partially refracted at the boundary surface, and partially reflected. However, if the angle of incidence is greater  than the critical angle ( the angle of incidence at which light is refracted such that it travels along the boundary of two media involved) then the light will stop crossing the boundary altogether and instead be totally reflected back internallyto the first medium. This can only occur where light travels from a medium with a higher refractive index to one with a lower refractive index.
						

							Hi C Kartik

Total internal reflection is an optical phenomenon that occurs when a ray of light strikes a medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface. If the refractive index is lower on the other side of the boundary, no light can pass through and all of the light is reflected. The critical angle is the angle of incidence above which the total internal reflection occurs.

Regards 
Bibhash Kr jha