Measure the Speed of Light with Chocolate & a Microwave

After presenting our Story of Light talk to the Mayo Dark Skies Festival on 31st Oct, we wanted to give details on how to measure the speed of light with a bar of chocolate and a microwave oven. A Microwave in science is an electromagnetic wave with a wavelenght between 1 mm to 30 cm. In your oven these microwaves heat up the water molecules in the food.

Microwaves travel at the speed of light or the same speed as visible light. There are two things to know about a wave its Wavelength, which is how long it is and the Frequency, which is how many waves pass a point per seconds.


If the wavelength is 1 mm and the frequency 300 GHz (which is 300,000,000 waves per second). If the wavelength is 30cm then the frequency is 300 times smaller at 1 GHz (which is 1,000,000,000 waves per second).

Find the Frequency of your microwave oven.

The frequency of your microwave is usually on the back of the oven. If your microwave is built in and you can’t get to the back of our microwave, you can take the frequency above of 2450 MHz.

When the chocolate is placed in the microwave, it only melts in spots and not in others. The reason is that the microwave is heating up only the places where the wave passes through the chocolate. This usually doesn’t matter as the food rotates but we have a stationary bar of chocolate so it shows the wavelength or I should say half the wavelength.

Measure the wavelength of microwaves.

How do you measure the Wavelength.
1. Remove the turntable of the microwave, so that the plate in the oven can not rotate.
2. Place a large bar of chocolate on a plate in the middle of the microwave.
3. Turn on microwave for around 30 seconds.
4. Take the chocolate out of the oven and you should see parts of the bar have melted and other parts have not.
5. Measure the length between the melted spots, the wavelength is twice this distance in meters. If you measure 6 cm between the spots then the wavelength is 0.06 m by 2 = 0.12 m.

The wavelength is twice the length between melted spot.

So you can see that you are very close to the speed of light. If you could make a more accurate measurement of the length and in the past I measured it to be 6.1 cm this gives a speed of light of 2,989,000 Km/s. Not bad for a bar of chocolate and a microwave oven.

Of course you will have to eat the chocolate afterwards and I would recommend strawberries with the melted parts. This experiment would with pizza or cheese on toast. I think that this experiment should be on Leaving Cert Physics syllabus or high school science syllabus.