Total Internal Reflection

We've looked at light going from air (or a vacuum) into water (or glass, etc.). Now imagine it the other way around: going from water into air. Clearly, the light ray will be bent AWAY from the normal line. Several rays are shown in this image - the normal lines are indicated by a dotted line.

As the light ray hits the water surface at a greater and greater angle, eventually the light ray can NOT leave the water. That is, it "skates" along the surface (angle equal to 90 degrees). Any angle greater than this so-called "critical angle" results in the light reflecting BACK INTO the water. This phenomena is called: TOTAL INTERNAL REFLECTION

It is the secret behind fiber optics.