# The speed of light is 392,000,000m/s?

**You don’t always need a big laboratory for exciting experiments... You can measure the speed of light using a microwave oven and a bar of chocolate.**

In the seventeenth century, the Italian physicist Galileo Galilei thought up a simple experiment to measure the speed of light. Give two observers a lamp and make them stand a large distance apart. The first observer flashes his lamp and at the same time the clock is started. As soon as the second observer sees this flash they flash back with their lamp. When the first observer sees this flash he stops the clock.

So far so good. In 1667 – Galileo had been dead 25 years – the experiment was carried out. In the first experiment a time of approximately one second was measured. The observers then moved further and further apart, however each time one second was measured.

Of course, that one second is not the speed of light, but the observers’ reaction time. However, as the time measured remained the same irrespective of the distance between the observers, it was fairly safe to claim that the speed of light must be *Very Very Fast*, otherwise they would have noticed a change during the experiment. So how fast is the speed of light? They concluded it had to be somewhere between ten thousand kilometres per hour and infinity.

You can do better than that! All you need to measure the speed of light at home is a microwave oven, a ruler and a chocolate bar. The idea is simple: the characteristics of any wave motion can be expressed as speed = frequency x wave length. Imagine you are standing on a beach and that a wave crashes on the shore every ten seconds. If the distance between two waves is exactly ten metres, you can calculate the speed of the seawater: v = 0.1 Hz x 10m = 1m/s.

Microwaves in a microwave oven travel at the speed of light and the frequency of the device can usually be found on the back of the oven (almost always 2.45 GHz). To measure the speed of light, you only need to determine the wavelength, and that’s where the bar of chocolate comes in.

Remove the turntable from the oven and place the chocolate bar on a piece of paper towel in the middle of the oven. Now turn the oven on for forty to sixty seconds at full power. You should see the chocolate starting to melt at certain points sooner than others. These are the points where the microwaves pass through the chocolate bar. The distance between two of these melting points is half the wavelength of a microwave.

The rest is pretty straightforward. Multiply the measured distance by two and insert the numbers in the right places in the formula. During a first experiment in the *Cogito ergo boom!* lab we measured a distance of approximately eight centimetres. This resulted in a speed of light of 392,000,000 metres per second, more than 30% faster than the actual speed of light of 299,792,458m/s.

Only after repeating the experiment several times, and not being too strict in our interpretation of the results, did we eventually arrive at a distance of 61 millimetres between two melting points. This resulted in a speed of light of 298,900,000m/s.

The question that now arose was how can you make the measurements more accurate? ‘Use many different kinds of chocolate,’ was the solution offered by one of our *Cogito ergo boom!* laboratory assistants. A logical suggestion. The melting points change depending on the ratio between cocoa and cocoa butter in the chocolate. It is plausible that certain types of chocolate will produce more accurate measurement points.

A thin layer of chocolate sprinkles is another option. This has the advantage that the microwaves do not need to pass through a thick layer of chocolate and also that the heat will spread out from the melting points less easily. The disadvantage is that there’s less chocolate left to eat at the end of the experiment!

Author: Ernst Arbouw

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Last modified: | 12 April 2021 1.12 p.m. |