Determination of the Speed of Light With a Chocolate and a Microwave Oven

As we know from the theory of relativity, the speed of light is constant regardless of the frame of reference. Suppose someone on earth switches on a flashlight and the light travels with the speed of light c, then a moving observer (for example sitting in an airplane) hurrying after the light strangely measures the same speed of light c.

Olaf Römer was able to estimate the speed of light as early as 1676 using the moons of Jupiter. Their entry into Jupiter's shadow was delayed more and more over the course of half a year and then occurred earlier and earlier in the other half of the year. The reason was the increasing or decreasing time required for light to travel from Jupiter to Earth.

In 1848, Hippolyte Fizeau was able to determine the speed of light using gears. He sent light through the gaps in a gear onto a distant mirror. On their way back, however, the cog wheel had already moved a little further. For this reason, the reflected light could only get back to the observer through the next gap in the gearwheel at a certain speed.

But there is a much simpler method of determining the speed of light. All you need for these are a large chocolate bar, a microwave oven, and a ruler.

A microwave oven generates microwaves inside to heat the food. Just like light, these microwaves are electromagnetic waves that propagate with the speed of light c. If these microwaves hit the inner walls of the microwave oven, they are reflected. However, the emitted and reflected waves interfere and a so-called standing wave is created. This standing wave seems no longer to wander, but stays where it is. So-called vibration nodes and antinodes are formed. The standing wave does not oscillate at all at the vibration nodes, but maximally at the antinodes. The distance between two adjacent antinodes corresponds to exactly half a wavelength!

For this experiment, however, we still need the wave equation, which links the quantities of light speed c, frequency f and wavelength lambda. Consider a transmitter that oscillates at frequency f. The wave emanating from it propagates in space with the speed of light c. After exactly one period Tau, the wave has spread by exactly one wavelength lambda.

Plug this into the formula for the velocity v = s/t and get: c = lambda/Tau. However, 1/Tau corresponds to the frequency f of the transmitter. Hence the wave equation follows: c = lambda * f.

If I know the wavelength lambda and the frequency f, I can calculate the speed of light c!

As said, all we need for this simple experiment is a large chocolate and a microwave oven. The chocolate is placed in the microwave oven. To do this, however, the turntable of the microwave oven must first be removed. The chocolate must not rotate in the microwave oven. Then switch on the microwave oven for a short time. Attention: Please only switch on the microwave oven for a short time, otherwise you will end up with a mess similar to mine...

The chocolate primarily melts at the antinodes. If you take the chocolate out of the microwave oven again, you can see melted spots. These have a certain distance from each other, which corresponds exactly to half the wavelength of the microwaves. In my case, the distance between the melted spots was exactly 6 cm. The wavelength lambda is therefore exactly 2 * 6 = 12 cm = 0.12 m.

The frequency f of the microwaves is also known, it is 2.45 GHz. Now insert this into the wave equation c = lambda * f: c = 2.45 * 10^9 * 0.12 = 2.94 * 10^8 m/s. This value is very close to the actual value of the speed of light, which is c = 299 792 458 m/s.

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