The phenomenon in which light is trapped (totally internally reflected) in one medium, because the refracted ray cannot exist (mathematically, according to Snell's Law, the sine function becomes greater than 1.0). Occurs when light traveling from a denser medium to a rarer medium strikes the interface at an angle of incidence greater than the critical angle for that interface. When this condition is met, light cannot be transmitted through the interface and so is all reflected back into the original material. The image to the left shows TIR just starting to occur, as the angle of incidence equals the critical angle. The critical angle and total internal reflection are covered on this page (and the next one) of this module. Not to be confused with total reflection.