Thursday, January 13, 2011

Bending Light

Lecture 14 of The Joy of Science describes the properties of Electromagnetic Radiation, including light. Light waves can be made to bend as they pass through substances of different densities, like air and water.

One experiment I always wanted to try was using a stream of water like a fiber optic cable to bend light. We saw a demonstration of this at the Corning Museum of Glass last summer. The exhibit at Corning got cool effects by varying the stream of water from a steady flow to drops.

Our version was made using an empty soda bottle and a laser pointer. It works best at night, or in a room that can be darkened. Here is what we did:


laser pointer (I got a couple different kinds from the supermarket, about $6 each)
stand for laser pointer (we tried poking it through a foam cup and laying it on a stack of coasters) 
rubber band
clear empty soda bottle with cap
permanent marker
metal skewer
masking tape

  1. Fill the bottle with water and put a few drops of milk in to make it slightly cloudy. You can also try mixing in a little powdered milk or corn starch.
  2. Wrap the rubber band around the laser pointer so it is pressing the "on" button continuously.
  3. Set up the laser pointer so it is lying horizontally about 3 or 4 inches from the surface of the counter. Put it next to the bottle and adjust until you see the line of light passing through the milky water.
  4. With the marker, make a dot where the light beam touches the far side of the bottle. Pour out all the water.
  5. Heat the metal skewer in the flame (such as a stove burner) until it is very hot. Use the tip to melt a small hole in the soda bottle at the dot.
  6. Cover the hole with a piece of masking tape. Put the cap on tight.
  7. Fill the bottle again with milky water. Place it on the counter next to the sink. Set up the laser pointer so it is aimed through the bottle at the hole. Take the tape off to see if the light hits the hole. (The water won't come out until you loosen the cap -- try it!)
  8. When everything's arranged, loosen the cap until you get a stream of water pouring into the sink. Adjust the light or walk around the bottle until you can see the light in the curved stream of water.

What's Happening?

The laser beam is "trapped" inside the water because of Total Internal Reflection. As the light tries to pass from the more-dense water to the less-dense air, it bends. (This is called refraction.) At a certain point the light bends so much that it is bounces off the surface of the water. (This is called reflection.) In the narrow column of water, the light wave continues bouncing off the boundaries of the stream of water but cannot pass through into the air.

The same principle applies with a piece of glass or plastic fiber optic cable. We have some lying around somewhere. I'd like to try using it in an art project, but with LED bulbs instead of lasers....