DIY Science: Calculate the speed of light

  • by National Office
  • 16 August, 2020
DIY Science: Calculate the speed of light

What you need:

  • a microwave oven
  • two blocks of chocolate (about 4 mm thick)
  • a plate, three ramekins and a ruler

What to do:

  1. Look on the oven’s compliance plate (inside the door or on the back) and note the microwave frequency (it’s usually 2450 MHz).
  2. Remove the turntable from the microwave.
  3. Put three ramekins on the floor of the oven, so they can hold a dinner plate up away from the spindle.
  4. Place the chocolate blocks next to each other on the plate and put it in the oven. Ensure it won’t rotate.
  5. Run the microwave on low for 30 seconds or so until the chocolate starts to melt in separate spots.
  6. Lift up the unmelted chocolate and remove it, leaving behind pools of melted chocolate.
  7. Measure the distance between the centres of two of the pools (see photo).
  8. Replace the turntable and melt all of the chocolate in a bowl to enjoy with fresh fruit.

Safety note:

Use microwave oven safe crockery and don’t overheat the chocolate. Beware of hot spots in the food.

What’s happening?

Microwaves are a form of light. Knowing the wavelength and frequency allows us to calculate the speed of light fairly accurately, even in a kitchen.

When the food doesn’t rotate the microwaves warm the food unevenly, and the hot spots correspond to the crests and troughs of the lightwaves. These are half a wavelength apart. We double the distance to get the wavelength.

Calculate the speed of light:

speed of light = wavelength x frequency
= 2 x measured distance x frequency
= 2 x 0.060 m x 2450 000 000 Hz
= 294 000 000 m/s (the actual speed is 299 792 458 m/s)

What speed did you get?

Share this article: