Ok, since I'm a student of physics/optics etc, here's the explaination. When light waves hit the surface of a transparent object, some pass through, some reflect, obviously. However, here's the deal, when the wave passes through an object the refraction index determines how quickly the light can travel (or any transverse wave) in the material. For physics sake and the sake of the explaination this will be called n. n determines the final angle the light will take based on its initial angle after it enters or exits the surface. This works as follows Sin(angle)initial / Sin(angle)final = n. Then you can simply solve for n in order to get the angle ie arcsin(angle) final = Sin(angle)initial * n. Which gets converted to Angle final = Sin(sin(angle initial)) * n. Now, when you enter glass the light gets bent towards the normal line (the perpendicular bisector of the surface. IE a virtual line drawn coming straight out of the surface) This causes a refraction index that is greater than one, water is 1.33, air is ~1.00 as is a vacuum. NOW we get to total internal refraction. Ok, here's when and why this happens, when a light beam is traveling from a surface with a high refraction index into a surface with a low refraction index (ie water into air) the light is bent AWAY from the normal line. At a certain incident angle the light will bend so far that it will actually become paralell to the surface and therefore will not escape the surface, even greater angles will result in even more internal reflection. However, that is the point of total internal reflection. Now, in materials with higher indexes of refraction the angle this requires is much less, as seen through the formula above. This is why diamonds sparkle so much more than cubic zirconia, because the refractive index is so much higher. Also why diamonds are cut at certain angles to project light through the top faces thus making it appear brighter than it would be otherwise. Pretty cool stuff, I'd draw you a diagram, but I'm really lazy.