Question

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

Answer 1

The angle of incidence in the denser medium corresponding to which the angle of refraction in the rarer medium is 90°  is called the critical angle. 

A light beam bends away from normal when it moves from a denser medium (water) to a rarer medium (air).

∠r ∠i as shown by AB, BD.

Hence, as we raise the angle of incidence, ∠r rises in tandem with AE. A stage is reached when the angle of refraction becomes 90°, or parallel to the separation surface, as the angle of incidence increases. Then ∠i is referred to as the crucial angle.