How to... measure the speed of light with bread, butter and a microwave!
Elliot Howe | 27 March 2017

Yes, you can actually measure the speed of light with bread, butter, and a microwave; approximately 3x108 metres per second.


The equation E=mc2 is a value loved by many. Why? Because E=mc2 explains why nothing can travel faster than the speed of light. This equation shows that m=e/c2, which implies that the higher the energy of a particle, the more mass it will gain.


Confused? Let me explain the theory. As a mass travels closer to the speed of light, it gains mass; therefore- naturally - the object will slow down, because more work will have to be done to move the heavier object. Imagine pushing a boulder, then a tennis ball. The boulder will require more work to push than the tennis ball because the boulder is much heavier. Don't worry about the interchanging use of mass and energy, good old Einstein realised that energy and mass were essentially the same thing. The only thing which has no mass is light; therefore NOTHING can travel faster than light.


Now, after the theory, how can we possibly measure the speed of light using bread, butter and a microwave? Start by taking the turntable out of the microwave and spread your butter onto three or four pieces of bread. Place these pieces of bread onto the base of the microwave (without the plate) and turn on the microwave. Leave the microwave on for about 20 seconds, until the moment you can see areas of the bread where the butter is melting. Turn off the microwave and measure the distance between two places where the butter has melted. Multiply this value by two.


There is a valid reason that we did that. A microwave is a light wave (remember the EM spectrum?) Like all light waves, a microwave has a frequency and a wavelength. In a microwave, the waves are emitted from one side and travel across to the other side. As they reach the other side, they get reflected back to the other side and so on. A standing wave formation is formed. The wave stops moving, because two waves approach each other from opposite sides and interfere with each other. As they interfere with each other, there will be points where the waves completely cancel each other out - a low amplitude - and points where they will not cancel each other out - a high amplitude. The points of high amplitude mean a high energy and points of low amplitude points of little or no energy. When the butter melts, this is at a point of high amplitude. Therefore, the distance between two places where the butter has melted is equivalent to half a wavelength, so if we multiply the distance by 2, we get a value for the wavelength of the microwave.


Now, we need to obtain a value for the frequency of the microwave, which should be found at the side or back of the machine. Once we have a value for the wavelength and frequency we can measure the speed of light. Using the equation E=hf - E being energy, h being Planck's constant (a fixed number for any calculation involving this equation), and f the frequency - we can find the energy of a microwave.


With this value of energy, we can then use the equation E=hc/λ (λ being the wavelength) to obtain a value for c - the speed of light.


There you have it. Again, another reason why science is cool. Using bread and butter, not only can you make a delicious pudding, but you can work out the speed of light.

James Routledge 2016