What is the critical angle?

The critical angle is the angle of incidence in a denser medium at which the refracted ray just grazes the interface (90°), beyond which total internal reflection occurs.

Formula: sin(C) = 1/n

Example (Glass, n = 1.5): sin(C) = 1/1.5 → C ≈ 41.8°

Applications: Optical fibres, diamond sparkle, periscopes/binoculars, mirages.

The critical angle is the threshold angle of incidence beyond which light undergoes total internal reflection instead of refracting out of the denser medium.