refraction

Look at a straw standing in a glass of water and it seems to snap and bend at the surface, even though the straw is perfectly straight. A pool always looks shallower than it really is. Both illusions happen because light changes direction when it crosses from one clear material into another. That bending is called refraction, and it answers the question: why does light not just keep going straight when it passes from air into water or glass?

The reason is that light travels at different speeds in different transparent materials, fast in air, slower in water, slower still in glass. When a ray meets the boundary at an angle, one edge of the beam reaches the slower material before the other edge does, so the beam pivots, like a marching band wheeling when one side takes shorter steps. Precisely: when light enters a material where it slows down, it bends toward the normal (the perpendicular to the surface), and when it speeds up on the way out, it bends away from the normal. The exact amount of bending is given by Snell's law, n1 sin(theta1) = n2 sin(theta2), where each n is the index of refraction of a medium. Only the direction changes at the boundary, the frequency (and hence the colour) stays the same, while the speed and wavelength change.

Refraction is the working principle behind eyeglasses, camera lenses, magnifying glasses, and the lens in your own eye, all of them steer light by bending it at curved surfaces. An honest caveat: refraction happens only when light meets a boundary at an angle. A ray that hits the surface head-on, straight along the normal, slows down but keeps going in a straight line with no bending at all.