the straw above the water line doesn't match up visually with the straw below the water line and therefore appears broken

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.

magnifying glass making things flowers larger than they are

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.

Try this experiment to see how different objects refract light.

What You'll Need

mirror, comb, flashlight, various bottles of liquids needed
  • 1 flashlight
  • 1 wide-toothed comb
  • 2 clear bottles or glasses
  • Water
  • Cooking oil
  • Construction paper (optional)

What To Do


Place the flashlight on the table and lean the comb against it.

a flashlight with a comb placed in front

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.)

flashlight turned on in the dark with light beams coming through gaps between teeth of comb

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?
a glass jar of oil in front of the light beams coming through the gaps between the teeth of the comb

Try This!

a glass jar with water in front of the light beams coming through the gaps between teeth of comb

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?

Image Credits:

Magnifying glass by Jade87 from Pixabay; all other images, courtesy of AMNH