Total internal reflection is a complete reflection of a ray of light within a medium such as water or glass from the surrounding surfaces back into the medium. It only occurs when both of the following two conditions are met:
- A light ray is in the more dense medium and approaching the less dense medium.
- The angle of incidence for the light ray is greater than the so-called critical angle.
The critical angle is the angle of incidence, for which the angle of refraction is 90°. If light enters a denser medium from a comparatively rarer medium, then the direction of light changes and the light ray bends towards the normal.
One can witness an effect by opening his or her eyes while swimming just below the surface. If the water is still, the surface outside the critical angle appears mirror-like, reflecting objects below. The region above the water cannot be seen except overhead. The hemispherical field of view is compressed into a conical field known as Snell’s window, whose angular diameter is twice the critical angle.