The speed of light isn't always the same. It actually slows down when it moves through some transparent materials, like glass or water. When light slows down, it changes direction. This "refraction" of light is the reason a straw in water looks bent or broken and why objects viewed through a glass bottle appear distorted.

In the same way light reflects differently off different surfaces, it also refracts differently depending on the shape of the material. This can make refraction very useful. For example, the curve of eyeglasses directs light rays into the eyes more effectively. Magnifying lenses also use refraction: the convex lens bends the light rays so the image appears larger.

	1 flashlight
	1 wide-toothed comb
	2 clear bottles or glasses
	Water
	Cooking oil
	Construction paper (optional)

	Place the flashlight on the table and lean the comb against it.
	Turn the flashlight on and turn off the lights in the room. Notice the light beams that are shinning through the comb's teeth. (NOTE: If you cannot see distinct, individual light beams, try wrapping a piece of construction paper around the end of the flashlight to extend its lens a little. This will help direct and focus the light beam.)
	Fill the small glass bottle or glass halfway with water and place it in front of the beams of light shining through the comb's teeth. What happens to the beams of light? Do the beams of light change direction? A focal point is where beams of light meet. Where is the focal point?
Now fill your glass bottle or glass with cooking oil instead of water and try the experiment again. Are the results the same? Do the light beams refract differently through the cooking oil than in water? Is the focal point the same?