[Video] Measure Speed of Light Using Microwave

By Anupum Pant

Microwaves are a part of the electromagnetic spectrum. That means they travel at the speed of light. And since it’s known that most commercial microwaves use a frequency of 2450 MHz or can be found on the user manual of your microwave, the speed of light can be calculated using a very simple experiment. Involving just you, your microwave, a pizza and a ruler. You could even use chocolate or marshmallows to do this.

To find the speed of light, all you need to know the frequency and the wavelength because frequency multiplied by the wavelength of an electromagnetic wave gives you the velocity of light. The wavelength of a microwave being used to heat your pizza is fairly easy to calculate. Here’s how:

Make sure the rotation plate is stopped for this cook and then put in a large pizza on a flat microwavable dish. Start the microwave in the lowest power for a long time and keep looking inside. You’ll see that the cheese will start melting unevenly. At that moment it starts melting, switch it off and take the pizza out. Now measure the distance between the centres of molten cheese points. It usually comes to about 6 cm. Multiply it with two to get the wavelength and then finally use the frequency to find the speed of light.

Velocity = Frequency ´ Wavelength

This works because the microwave heats pizza the most, at places where there is a peak in the electromagnetic wave. All the points where the wave seems to be not moving are the places where the pizza heats the least. Turns out, the two nearest peaks (the highest point and the lowest point of the wave, where the cheese first melts) in any electromagnetic wave are separated by a distance of half the wavelength. It makes sense when you see the diagram below. This is the reason most microwaves use a rotating table to heat up your food evenly.


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