Write the definition of critical angle.

(N/A) The critical angle is defined as the angle of incidence in a denser medium for which the angle of refraction in the rarer medium is $90^{\circ}$.
When light travels from a denser medium to a rarer medium,as the angle of incidence increases,the angle of refraction also increases.
At a specific angle of incidence,the refracted ray grazes the interface between the two media,making the angle of refraction $90^{\circ}$. This specific angle of incidence is known as the critical angle ($C$ or $i_c$).