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Bend a Pencil (Using Light)

Bend a pencil using light, and find out how this illusion works.

You Will Need:
- Water
- Pencil
- Clear Cup

What to Do:

  1. Fill the glass with water so it is about 2/3 full.
  1. Hold the pencil straight up and down in the glass so that it is touching the bottom.
  1. Take a look from the side. How does it look? It probably still looks straight.
  1. Now let the pencil lean to the side of the cup. You don't need to hold it anymore. The top part of the pencil should still be sticking out of the water.
  1. How does it look from the side this time? It's bent?! Whoa... how did that happen?

What's the Science?
An interesting property of light called refraction just took place. When light enters the water, it can't move as fast and it has to slow down slightly. It's kind of like how if you are walking, you can walk at a normal speed but if you walk in water, you can't walk quite as fast.

If light from the image enters the water straight, then the image looks normal - which was what you originally did when the pencil was straight up and down. If the light enters the water at an angle, then the change in speed between the open air and water causes the light beam to bend away from its original path. When the pencil was at an angle, the image was at a bigger angle in the water than in the air and made the pencil look like it was bent.

You can try this with other liquids and objects too. They may refract light images differently.

For more lighted fun, be sure to check out...

The Exploratorium Snacks About Light

And the Optics Cart in the Creative World when you're visiting the California Science Center.

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