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Bouncing Lights


You and Your Students!

Script By

Vicki Cobb, Education World Science Editor

Show-Biz Science is scripted by popular children's book writer Vicki Cobb. Click to learn more about Vicki or to read a brief synopsis of her philosophy of teaching science.

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A diamond’s sparkle and cable TV have something in common.


Light, Technology

Props Required

  • a rectangular clear glass baking dish
  • a flashlight (or a laser pointer)
  • a clear glass measuring cup
  • a bowl of water

Setting the Scene (Background)

A diamond sparkles because light bounces off the inside of its back surfaces. (It also reflects off the top surfaces if held at the right angle.) A diamond is cut so that it has many surfaces at different angles to each other. When the diamond is moved, the back surfaces reflect light to your eyes and you see them as flashes. Diamonds are not the only things that reflect light internally. A glass baking dish also does the job. Internal reflection is the key to a diamond’s sparkle and fiber-optic cables.

The Plot

Act I

Place the glass baking dish on a level surface and shine the flashlight down on one rim of the dish. Look at the rim of the dish on the side opposite the flashlight. Move the flashlight back and forth along the rim and follow the light that appears on the rim of the other side.

Here's what's happening: Some of the light you shine through the rim of the baking dish reflects internally off the inside surface of the side of the dish. This reflected light hits the other surface and again is reflected internally bouncing back and forth until it exits out the other end where your eye is waiting to see it. Internal reflection bends light around corners!

Act II

As you might expect, a glass measuring cup will bend your light beam just like the baking dish does. Hold the measuring cup in one hand while you send the light from the flashlight down through the rim of the cup. Put your eyes close to the rim and find the light coming up through the glass.

After you find the light, put the bottom of the cup into the bowl of water. See the light dim.

The internal reflection is influenced by the way light travels through the boundary between the glass surface and air or water -- all transparent materials. Water is not as transparent as air. More of the internally reflected light is lost at the water-glass boundary than at the water-air boundary and so less light ends up at your eye.

Behind the Scenes

Optical fibers are very thin flexible rods of glass surrounded by another transparent material. Light is sent into the fiber and it bounces back and fourth all the way to the other end, which can be hundreds of miles away! (You can see fiber optics at work with some holiday decorations.) Pulses of light in a fiber-optic material can carry the same kind of information that is transmitted as electrical pulses in a copper wire. That information can be telephone conversations or television shows or data from computers and fax machines. A conventional copper wire can carry a few million electrical pulses each second; an optical fiber can carry as many as 20 billion light pulses per second! That extra capacity is what makes hundreds of cable channels and the Internet possible.

See more experiments in Light Action! Amazing Experiments With Optics by Vicki Cobb and Josh Cobb, illustrated by Theo Cobb.

Article By Vicki Cobb
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