Define critical anhle or total internal reflection.

Total internal reflectionis a phenomenon that happens when a propagatingwavestrikes a medium boundary at an angle larger than a particularcritical anglewith respect to thenormalto the surface. If therefractive indexis lower on the other side of the boundary and the incident angle is greater than the critical angle, the wave cannot pass through and is entirelyreflected. Thecritical angleis theangle of incidenceabove which the total internal reflectance occurs. This is particularly common as anoptical phenomenon, where light waves are involved, but it occurs with many types of waves, such aselectromagnetic wavesin general orsound waves.
When a wave crosses a boundary between different materials with different kinds of refractive indices, the wave will be partiallyrefractedat the boundary surface, and partially reflected. However, if the angle of incidence is greater (i.e. the direction of propagation or ray is closer to being parallel to the boundary) than the critical angle – the angle of incidence at which light is refracted such that it travels along the boundary – then the wave will not cross the boundary and instead be totally reflected back internally. This can only occur when the wave in a medium with a higher refractive index (n1) hits its surface that's in contact with a medium of lower refractive index (n2). For example, it will occur with light hittingairfrom glass, but not when hitting glass from air.