Total internal reflection is an optical phenomenon that occurs when light traveling in a denser medium hits the boundary with a less dense medium at an angle greater than the critical angle. In this case, all the light is reflected back into the denser medium, with no light passing through to the less dense medium.

Total internal reflection definition: TIR (total internal reflection) is an optical phenomenon in which waves, such as light, are entirely reflected when they reach the boundary between two mediums, such as air and water, under particular conditions. When waves travelling in one medium collide with another medium with a higher wave speed (lower refractive index), they are not refracted into the second ("external") medium but are entirely reflected back into the first ("internal") medium.