You (yes you!) can measure the Speed of Light

with a Microwave Oven and Marshmallows!  

Teachable Topics:

  • Electromagnetic Spectrum
  • Frequency & Wavelength
  • Relativity


Microwaves get their name from the part of the electromagnetic spectrum in which they operate. Microwaves cooks your food using electromagnetic waves with a wavelength between 1mm and 30cm (Click here for more info).

Using a microwave operating at a known frequency (usually found on the back – ours was 2450MHz), it is possible to measure the speed of light. The wavelength your microwave uses can be seen by placing one layer of marshmallows on a plate as to cover most of the plate and microwaving them until they just start to melt. When you remove the plate, you will find that the marshmallows have melted in some areas more than others. Measure the distance between these melted spots, you should come up with a value of the microwave’s wavelength - we found it to be approximately 12cm (see Figure 1).


Marshmallow Speed of Light 
Figure 1: Distance between melted spots on a plate of marshmallows.

With these two values, you can use the equation:

 c = fl

where c is the speed of light, f = frequency (in Hz), and l is the wavelength (in meters).

Using our values of n = 2450 x 106 Hz and l = 0.12m we get a value for the speed of light to be c = 2.94 x108 m/s.


  • microwave with labeled frequency
  • bag of marshmallows (can also use shredded cheese or chocolate chips)
  • plate (must be at least 13cm in diameter)


  • Place an array of marshmallows onto the plate, keeping as few empty spaces as possible.
  • Place the plate into the microwave and turn it on, watching the marshmallows for any melting.
  • Allow the plate of marshmallows to cook until you see some start to melt. We found this took about approximately 30 seconds, but this could vary depending on microwave power.
  • Remove the plate from the microwave and measure the distance between the melted spots. This will be your value for l. Using the frequency labeled on the back of your microwave (your f value), calculate the speed of light!


  • follow the safety instructions on your microwave - no metal, soft plastic, etc is to be placed inside
  • do not eat the marshmallows if the microwave has been used previously for other experiments!