This simple activity has surprising results! We’re going to bend light using plain water. Light bends when it travels from one medium to another, like going from air to a window, or from a window to water. Each time it travels to a new medium, it bends, or refracts. When light refracts, it changes speed and wavelength, which means it also changes direction.

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Materials:

• Red and green laser
• Paperclip
• Index card
• Tape
• Rubber band
• Water glass

1. Open the paperclip into an “L” shape, and tape it to an index card so the card stands up. This is your projection screen.
2. Use the rubber band to attach the laser pointers together. You’ll want them very close and parallel to each other. Place the rubber band close to the ON button so the laser will stay on when you put the rubber band over it.
3. Place the laser pointers on a stack of books and put switch them on with the rubber band.
4. Shine the lasers through the middle of an empty glass jar and onto the screen.
5. Put a mark where the red and green laser dots are on the screen.
6. With the lasers still on, slowly fill the container with water. What happened to the dots?
7. You can add a couple of drops of milk or a tiny sprinkling of cornstarch to the water to see the beams in the water.

Here’s a quick activity you can do if the idea of refraction is new to you… Take a perfectly healthy pencil and place it in a clear glass of water.  Did you notice how your pencil is suddenly broken? What happened? Is it defective? Optical illusion?  Can you move your head around the glass in all directions and find the spot where the pencil gets fixed? Where do you need to look to see it broken?

When light travels from water to air, it bends. The amount it bends is measured by scientists and called the index of refraction, and it depends on the optical density of the material. The more dense the water, the slower the light moves, and the greater the light gets bent. What do you think will happen if you use cooking oil instead of water?

So the idea is that light can change speeds, and  depending on if the light is going from a lighter to an optically denser material (or vice versa), it will bend different amounts.  Glass is optically denser than water, which is denser than air. Here’s a couple of values for you to think about:

Vacuum 1.0000
Air 1.0003
Ice 1.3100
Water 1.3333
Pyrex 1.4740
Cooking Oil 1.4740
Diamond 2.4170

This means if you place a Pyrex container inside a beaker of vegetable oil, it will disappear, because it’s got the same index of refraction! This also works for some mineral oils and Karo syrup. Note however that the optical densities of liquids vary with temperature and concentration, and manufacturers are not perfectly consistent when they whip up a batch of this stuff, so some adjustments are needed.

Questions to Ask

1. Is there a viewing angle that makes the pencil whole?

2. Can we see light waves?

3. Why did the green and red laser dots move?

4. What happens if you use an optically denser material, like oil?

Click here to go to next lesson on Diffraction

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