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:
