If you’re looking for something to do on a random Saturday morning, why not try measuring the speed of light with a microwave and a bar of chocolate? One of my colleagues told me of this experiment when I was looking for a good way to introduce the equations of light to a class.

You need a bar of chocolate at least 10 cm in length – one of the 100 – 150 g bars work well, and you need a microwave with the turntable removed. You don’t want the chocolate to rotate at all. Put the chocolate on a microwave safe plate and microwave on high for 8 – 10 seconds.

When the chocolate comes out, use a butter knife to gently tap the chocolate to find the soft spots. You need to find two and they should be around 7 – 10 cm apart. Mark them clearly, and measure the distance between them.

You also need to find the frequency of the microwave, either from the manual or a sticker on the back.

Microwaves are standing waves within the microwave chamber which means they will cut through the chocolate twice for every wavelength. The distance from soft spot to soft spot is therefore half a wavelength. The speed of light is equal to the frequency times the wavelength and so all you need to do is take the frequency in megahertz and the half wavelength in centimetres, convert them into sensible units and figure out the speed of light. I measured 8 cm between the softspots so with the microwave’s frequency of 2450 MHz, that gives the speed of light as approx. 4 x 10^8 m/s, compared with the standard approximation of 3 x 10^8. I think getting the right order of magnitude is not bad going at all!

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This qualifies as seriously cool.

As a statistician, I feel the need to point out that repeating this experiment lots of times and taking the average will lead to a better estimate. And, just by chance, chocolate goes on sale tomorrow.

Yes I’d agree entirely with that 🙂

Fantastic! I might try this with my youngest nephews when they get a wee bit older. Right now there is no way they’d let me get to the measuring phase without grabbing the chocolate and running away with it. The older ones are too cool to care 🙂