Explain the term critical angle with the aid of a labelled diagram.

When a ray of light passes from a denser to a rarer medium, at a certain angle of incidence i = C, the angle of refraction becomes 90°.

This angle C is called the critical angle.

Thus, critical angle is defined as the angle of incidence in the denser medium corresponding to which the angle of refraction in the rarer medium is 90°.