Great question! Light actually moves in waves. The wavelength, the distance from one wave "crest" to the next, determines the color of the light. If you look closely, however, you can see that the light spreads out just like other waves do- say a water wave passing through a little passage. This becomes particularly noticeable if you look at light that has passed through a narrow slit. However, the wavelength of visible light is so small that you can make beams which travel in straight lines without spreding out very much.
Light changes direction when it bumps into some sort of change in material, as we describe below.
(1) Reflection. Reflection is what happens when light bounces off of something. This is what you see when you look in the mirror. This is also what you see when you look at most things around you, not counting light bulbs, fires, and other actual sources of light. In order to reach your eyes, light bounces off of the object that you're looking at, changing its direction. It can also bounce off of particles (like water droplets) in the air, which is why you may be able to see beams of sunlight on a foggy day.
(2) Refraction. Refraction is what happens when light passes from one medium (one type of stuff) into another. When this happens, the light will actually bend. (Actually, it usually splits in two - some of it reflects off the surface, and some of it keeps going through.) The light that goes through bends at the surface. The angle that it bends at depends on what the two media are. For example, light will bend a different amount when it goes from air into glass than it does when it goes from air into water.
This is a really good picture of light passing through a piece of glass. You can see that some of the light bends and goes through the glass (refraction), and some of the light bounces off of the surface of the glass (reflection).
-Tamara (some mods by mw)
(republished on 07/29/06)