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Use Thinking Putty to Confirm the Speed of Light

Use Thinking Putty to Confirm the Speed of Light

Thinking Putty allows us to perform some pretty neat home experiments. Did you know that with Hypercolor Thinking Putty and a kitchen microwave, you can estimate the speed of light?

[WARNING: Please do not place Thinking Putty in the microwave without parental supervision. Microwaved Thinking Putty can become very hot and cause burning when removed. Even adults should use extreme caution when placing Thinking Putty in the microwave or any heating unit.]

Here's what you'll need:
  • Hypercolor Thinking Putty
  • A piece of cardboard
  • Some plastic bags
  • A water filled mug (with no metallic ink or edging!)
  • An empty dish
  • A ruler that can measure in centimeters

"The universe itself keeps on expanding and expanding
  In all of the directions it can whizz
  As fast as it can go, at the speed of light, you know,
  Twelve million miles a minute, and that's the fastest speed there is."
- Monty Python's The Meaning of Life
Light moves fast. Real fast. This can make it hard to measure.

One of the first people to attempt measurement of the speed of light was Galileo. In the early 17th century, the general belief amongst natural philosophers (as scientists were often called) was that the speed of light was infinite; that is, light could travel any distance in no time at all.

Just as with many other important physical phenomenon, Galileo felt the need to challenge the accepted beliefs of his contemporaries. One of Galileo's great strengths was his ability to conceive experiments that might accurately test different theories. To measure the speed of light, he and his assistant each took a shuttered lantern to hilltops one mile apart. Galileo flashed his lantern, and the assistant was supposed to flash his own lantern as soon as he saw Galileo's light. Galileo would then time how long it took before he saw the light from the other hilltop. Then, simply, he could divide the distance by the time to obtain an estimated speed.

Unfortunately, the extraordinary speed at which light travels was inconceivable to Galileo (there was a reason that others thought the speed was infinite!). Today, we know that light travels at approximately 186,000 miles per second. It would have travelled the one mile (1.6 km) between the hills in 0.000005 seconds. A little bit quicker than Galileo could have been on the stopwatch!

Did you know that your common household microwave generates light? And I'm not talking about the little light bulb inside that goes on so you can watch your food cook! Your microwave contains a device called a magnetron which generates light that is invisible to the human eye. The color of this light is very specific.

Diagram of a MagnetronPhoto of a Magnetron

We generally think of water as being "clear" but water absorbs certain colors of light which are invisible to us. When the energy from this light is absorbed, it turns into heat (like blacktop on a summer day). So, the microwave creates light which is opaque to water, the water absorbs it, and your food gets hot! Neat!

What we see as colors are really different wavelengths of light -- light which oscillates at a varying speed. The "frequency" of a color is a way of defining the wavelength. When you tune your radio to 107.9 Mhz, you are listening to a color of light whose "wave" peaks and troughs 107,900,000 times per second.

When you see the color green, you are seeing light oscillating 550,000,000,000,000 times per second (550 Terahertz!). BTW, all the colors you can see with your eyes are between 384 Terahertz (deepest red) and 769 Terahertz (palest violet). Visible light is the tiniest sliver of all the potential colors of light!

At certain frequencies the spacing between peaks and troughs becomes distances we are familiar with in our daily lives: inches, centimeters, feet, meters, etc. Luckily for us, a microwave's light is just such a frequency.

You probably know all about the "cold spots" in your microwave. Many microwaves have turntables to mitigate this problem. These cold spots (and their associated hot spots) are areas where the microwave light consistently peaks or troughs (hot) and consistently is in between (cold).

If you place a Hypercolor Thinking Putty in the microwave, you'll be able to reveal these locations.

[WARNING: Please do not place Thinking Putty in the microwave without parental supervision. Microwaved Thinking Putty can become very hot and cause burning when removed. Even adults should use extreme caution when placing Thinking Putty in the microwave or any heating unit.]

The Thinking Putty will begin to change color in certain specific areas (hot spots). By measuring the distance between them, you'll have the distance between 1/2 wavelength of microwave light. Just multiply by 2 to obtain the full wavelength (see the diagram below if this sounds confusing).

To finish up is very simple. If you multiply the length of the wave by the number of times it travels that distance in one second (the frequency), you'll get the speed of light! Luckily, we know your microwave's frequency. It's usually written right on the back along with all the warnings and legalese. On Crazy Aaron's microwave, it's listed as 2400 Megahertz (aka 2.4Ghz aka 2,400,000,000 times per second).

So, first we flatten out our 1 lb ball of Thinking Putty onto a piece of cardboard. The plastic bags will help ensure an easy removal. Then, we put it in the fridge for a few minutes so it can cool off and every section can even out in temperature.

Flatten out Thinking Putty onto cardboardCool off the Hypercolor Thinking Putty by placing it in the fridge for a few minutes

Be sure to remove the microwave's turntable and place the dish upside-down over the motor. Now, we fill a coffee mug with water and place it on top of the dish. Then, place the puttied cardboard on top.

Note: We use the mug of water as a "sink" -- a place for all the excess microwave energy to collect as heat. This is good for your microwave -- trust me. Also, you'll get better results if your Thinking Putty is in the middle of the microwave and not sitting at the bottom. The mug makes a good stand.

Remove the turntable and tray from the microwave

Turn on your microwave and watch through the window. It shouldn't take more than a minute before you see little colored spots forming on the surface of the Hypercolor Thinking Putty. Stop the microwave once you start to see these spots.

Put dish and mug over turntableSpots starting to form inside microwave

Take the cardboard out of the microwave and measure the distance between the center of the spots. Your measurement should be in centimeters.

Our own test gave us a 6 centimeter distance between the center of the spots. That's 0.06 meters (100 centimeters = 1 meter).

Heated putty showing hot spotsMeasurement #1

Measurement #2

So, 0.06 meters times 2 times 2.4Ghz = 288,000,000 meters per second. Compare that with the more scientifically precise value of 299,792,458 meters per second and we're within 5%! Not too shabby!

This experiment is easy and fun but there are a few safety precautions that I must repeat:
  1. Do not put metal in your microwave. If you are going to try this experiment PLEASE remember to take the Thinking Putty out of its metal tin!
  2. Do not turn on your microwave with only a tiny little piece of putty in it. Your microwave needs to have something in it, a glass of water for example, to help absorb all the extra energy.
  3. Your microwave will make the Thinking Putty get hot! In the spots where the color has changed, the Thinking Putty can be VERY HOT...almost molten. This can burn you.
  4. Put your Thinking Putty on a piece of paper or cardboard so that you can avoid touching it during measurement and removal from the microwave. Wait until after it has changed back to the "cold color" before you touch it again.
Have fun!

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