Chocolate and the Speed of Light

Did you know? You can measure the speed of light at home in your kitchen, and you get a tasty treat at the end of it. That’s the type of experiment we LOVE here at Fun Science! Don’t worry, it’s a lot easier than you think – try some physics at home.

You will need:

  • A big bar of chocolate (Yum!)
  • 2 paper plates
  • A microwave
  • A ruler/tape measure
  • Calculator

Method:

  • Take the turntable out of your microwave and place a paper plate upside down where the turntable was.
  • Place the other paper plate the right way up on top of the first one.
  • Unwrap the chocolate and place it on the paper plate.

    Photo from Tea Time with Annie Kate

  • Turn the microwave on for 25 seconds.
  • Check the chocolate, you should see 2 spots where it has started to melt! If not, turn it over and you should see them.
  • Measure the distance between the melted spots using the ruler. This is the wavelength of the microwaves in your microwave! (Now it gets a bit math-sy, but don’t worry!)
  • Check the frequency of your microwave (this is how often the waves move up and down) – this should be on the door or the back of your microwave (usually 2.45GHz).
  • Multiply the wavelength (distance between the melted spots) and the frequency together and you will get the distance light travels in a second!
  • If you measured the wavelength in centimetres, you should get about 30,000,000,000 centimetres/second.
  • If you measured the wavelength in inches, you should get about 11,760,000,000 inches/second.
  • Eat the chocolate to celebrate!

The Science Bit:

Why paper plates? ‘Real’ or China plates absorb the radiation from the microwave and heat up, which would melt the chocolate all over, rather than just in the spots we want!

Why take out the turn table? The turn table is there to ‘even out’ the hot spots, so that your food doesn’t just cook in 2 spots, but that is what we want in this experiment!

How can microwaves tell us the speed of light? Microwaves are a form of light, and light is a wave! Frequency x wavelength = speed. So that’s how we found the speed of light!

If you didn’t get the exact number, but you were close, this is probably because of the size of the melted spots, they mean that we can’t quite measure the distance accurately. Is there a way you can think of to make it more accurate?

Thanks to Tea Time with Annie Kate for this tasty and fascinating experiment!