critical angle

The critical angle is the angle at which light from within a transparent medium just grazes the surface of the medium. In the diagram, the red beam is at the critical angle. Blue beams escape from the medium, at least partially. Green beams are totally reflected from the surface.

In optics, for a ray of light passing from a denser to a less dense medium (such as from glass to air), the smallest angle of incidence at which the emergent ray grazes the surface of the denser medium – at an angle of refraction of 90°.

When the angle of incidence is less than the critical angle, the ray passes out (is refracted) into the less dense medium; when the angle of incidence is greater than the critical angle, the ray is reflected back into the denser medium (see total internal reflection).