Define critical angle. Write two conditions for total internal reflection.

Explanation

Consider a point object in the denser medium. There will be cone of rays striking the interface. For a ray of light OA, it suffers normal refraction. For any angle of incidence less than critical angle such as ray OB, the refracted ray bends away from the normal. However for a particular angle of incidence called critical angle, the refracted ray of light grazes the interface. Therefore critical angle for the given pair of media and given wavelength of light may be defined as the angle of incidence in the denser medium for which the refracted ray is at right angles to the normal drawn at the point of incidence.
For a ray of light such as OD, whose angle of incidence is greater than the critical angle, there will be no refraction of light, instead the light bounces back into the same denser medium. This phenomenon of light is known as the total internal reflection.

Conditions for the total internal reflection.
(a) Object should be in the denser medium and viewed through rarer medium.
(b) Angle of incidence in the denser medium should be more than the critical angle for the given pair of media and wavelength of light.