Total internal reflection is a phenomenon that occurs when waves arrive at the interface (boundary) from one medium to another and are not refracted into the second medium, but completely reflected back into the first medium. This phenomenon is observed when light rays travel from a denser medium to a less dense medium. The angle of incidence must be greater than the critical angle for total internal reflection to occur. The critical angle is the angle of incidence that provides an angle of refraction of 90 degrees.

Some key points about total internal reflection include:

• Total internal reflection occurs when both of the following two conditions are met: the light ray moves from a more dense medium to a less dense medium, and the angle of incidence must be greater than the critical angle.
• The critical angle can be calculated from Snells law by setting the refraction angle equal to 90 degrees.
• Total internal reflection is important in fiber optics and is employed in polarizing prisms.
• Glass prisms can be shaped to produce total internal reflection and are employed in binoculars, periscopes, telescopes, and other optical instruments.

Total internal reflection is often demonstrated in physics classes through a variety of demonstrations.