## Refraction and Snell's Law

Consider the pencil of the second introductory exercise. The apparent bend in the pencil is caused by refraction. Light rays bend, or refract, as they move from one medium to another. Not surprisingly, the index of refraction is related to this effect.

Refraction occurse because a wavefront moving from air to water at an angle to the water's surface does not reach the water all at once. Initially, just one side of a wavefront reaches the water and slows down, as shown in the animation below. What happens next can be visualized by considering a marching band. Imagine a row of band members marching side-by-side. When they want to make a turn, the members at one end of the row walk slower than the members at the other end of the row. The row spins around, with the slow-walking members at the inner part of the turn. In a similar manner, a wavefront encountering the water bends, as shown below. Lenses focus images by refracting the light coming from the image. You may learn more about lenses later in your physics course.

When describing the bending of the wave, we measure angles from the normal to the surface, just as when considering reflection. The angles between the light ray and the normal to the surface in each medium are related to the indices of refraction in those media by Snell's Law:

n1 sin 1 = n2 sin 2

Here 1 is the angle of incidence between the light ray and the normal in the first medium. Similarly, the light ray makes an angle of refraction 2 with respect to the normal in the second medium. The first medium has an index of refraction of n1, and the index of refraction in the second medium is n2.
The picture we have drawn above is not completely accurate. Not all of the light striking a surface travels across the surface; some is reflected. You have probably seen this happen when you looked out a window. Often you can see reflections of objects inside the room at the same time as someone outside the window could see the objects. The light you see is reflected; the light the person outside sees has been refracted through the window. In this module, we are primarily concerned with the light that is transmitted and refracted, but we must remember that some light is reflected as well.