Snell's Law for Light and Students

Optics, the study of visible light, is one of the earliest branches of science.  An introductory physics class will usually highlight what happens when a light ray encounters a new medium.  That is, a light ray may be travelling in a straight line unperturbed through medium 1, air, when it suddenly encounters medium 2, a thick plane of glass.  Before the light ray strikes the boundary, it is referred to as an incident ray.

The incident light ray will divide itself upon striking the new medium: some of the ray will reflect, and some of it will refract.  The reflection of a light ray is simple - it is no different than what happens to a billiard ball when it strikes a band.  Just as a billiard ball rebounds off of the flat border with an angle equal to that which it struck with, the angle of the reflected ray is equal to the angle of the incident one.

However, light can also permeate the new medium (an ability that no billiard ball possesses).  The portion of the light that passes through the boundary, continuing its travels into medium 2, is referred to as the refracted ray.  As shown in the figure below, the angle of each ray is measured with respect to an invisible 'normal' line, which is drawn perpendicular to the surface.