Measure the speed of light with a chocolate bar

Project Description

Remember Einstein’s lovely little equation E = mc 2 – the one that wrapped up the Theory of Relativity? The c in that equation stands for the speed of light. Now you can calculate that astonishing speed with just a microwave, ruler, calculator and a chocolate bar!

Grades: 6 to 12+
Duration: 1/2 Hour – 1 Hour

Supplies: Microwave, Ruler, Calculator, Chocolate Bar (at least 5″ long)

Step 1


Visible light is just one example of an Electromagnetic Wave. Microwaves are a type of electromagnetic wave too, just like visible light waves. Microwaves also travel at the speed of light. If you measure how fast they are travelling, you should get a result close to the speed of light.

All light waves (whether micro waves, radio waves or visible light waves) have a wave length that is the distance between the crests of two waves as in the photo above.

Visible light waves are very, very (microscopically) short and Radio waves are very, very long. But microwaves are just the right length for us to measure in this fun way.

Once you know the length and frequency of a wave, it is just some simple math that gives you the speed.

c = λf where:

  • c = the speed of light = 300,000 km/s or 3.0 x 108 m/s
  • λ= the wavelength of light, usually measured in meters or Ångströms (1 Å = 10-10 m)
  • f = the frequency at which light waves pass by, measured in units of per seconds (1/s)

Step 2

Set up the microwave

  1. Look for and write down the FREQUENCY of the microwave oven that you are using. You will need the frequency to calculate the speed of light.
  2. Take the turntable out of the microwave. You need the chocolate to stay still whilst you heat it.
  3. Put a plate upside down over the thing that rotates the turntable (does that have a name? For now I’m going with ‘rotator’).
  4. Put your chocolate in the middle of the plate.
Step 3

Heat up the chocolate

Heat the chocolate until it starts to melt in two or three places. This should take about 20 seconds.

Step 4

Measure the wavelength of a Micro Wave

Take the chocolate out of the microwave – carefully! It will be hot. Measure the distance between the melted spots. The distance between the two hot (melted) spots is the distance between wave troughs and equals the wavelength of the microwave!

Step 5

Calculate the Speed of light

Now you know the wavelength you need to know the wave frequency. Wave frequency is how many times a wave bounces up and down in one second. This is 2.45 gigahertz in most microwaves. This means that a wave will move up and down 2.45 billion times per second.

If your microwave is a standard model, it will have a frequency of 2.45 gigahertz. This means that the microwaves move up and down 2.45 billion times per second. Check in your microwave manual if you’re not sure of the frequency.

Multiply the distance between the spots on the chocolate bar by two. Multiply that by 2,450,000,000 (2.45 gigahertz expressed as hertz).

Distance between two melted spots of chocolate x 2 x 2450000000 = z

What answer do you get for z? The speed of light is 299,792,458 metres per second.

Remember, if you measured the distance between the melted spots in centimetres, z will be in centimetres per second. To get an answer in metres per second, divide z by 100.

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