Before we can do much with refraction, we have to discuss Snell's law. Whereas the law of reflection tells us how light bounces off of objects, Snell's law tells us how it bends when it passes from one material to another. It is a bit more complicated than the law of reflection since it must include the indices of refraction of the two media. Snell's law is:[br][br][math]n_1\sin\theta_1=n_2\sin\theta_2.[/math]

As in the last chapter, all angles must be measured with respect to the surface normal. The equation tells us that the incoming incident angle of light in the first material times the refractive index of that material is equal to the refracted (outgoing) angle in material two times the refractive index of material two. [br] [br]A diagram of this is shown below. Light leaves point P, strikes the interface between the two materials at point O, and continues to point Q. It should be noted, however, that optics ray tracing diagrams are reversible. This means the light could just as well have originated at Q heading for point O, and it would still pass through point P to the left. This reversibility is a general property of optical systems.