Activity: Speed of Light

Talia's cheesy experiment

On SCOPE's Light episode, you saw Talia use cheese and a microwave to measure the speed of light in a kitchen. Here's how she did it. Watch the clip.

What you need

• cheese slices
• microwave
• ruler
• calculator

What to do

1. Start by covering the plate evenly with slices of cheese.
2. Before you put the plate in the microwave remove the turntable so it doesn't rotate when the microwave oven is switched on.
3. Put in the plate of cheese and microwave until the first signs of cheese melting (10 - 20 seconds usually).
4. Take the cheese plate out and examine it. There should be several hot spots over the cheese where there has been melting, whilst the rest will be still cold and solid.
5. Use the ruler to measure the shortest distance between hot spots. Do a few. The distance should be about the same.
6. This distance is half the wavelength of a microwave, so to get full wavelength, double your number.
7. Next find the frequency of the microwave. It should be written on it somewhere. Most are 2450 Megahertz, or 2450 million Hertz.
8. Almost there. Now just multiply the frequency by the wavelength and you have the speed of microwaves (which is the same as light).

Here is an example:

• 6 cm between spots, means a 12 cm wavelength. Or 0.12 metres.
• So 0.12 * 2,450, 000, 000 Hz = 294, 000,000 m/s or about 1 billion km/hr

Now that is fast!

What's happening?

Did you know it takes just eight minutes for light to travel 150 million kilometres?

So it is pretty cool that you can measure the speed of light using stuff from kitchen?

Microwaves and visible light rays are both forms of electromagnetic radiation. They have different frequencies, but travel at the same speed. Therefore the speed of the microwaves inside the oven is the same as the speed of light travelling from the Sun.

The actual speed of light in a vacuum is 299,792,458 metres per second. Your figure won't be exactly that, as light and microwaves slow down when they travels through things like air and cheese.

The experiment will also work with chocolate buttons or marshmallows instead of cheese.