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 beyond which rays of light passing through a denser medium to the surface of a less dense medium are no longer refracted but totally reflected. It is the smallest angle of incidence that yields total reflection.
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, where the hemispherical field of view is compressed into a conical field known as Snell’s window, whose angular diameter is twice the critical angle.