Science Lab / The Speed of Light

Tuesday, 31 March, 2015

It’s taken mankind over 300 years to pin down the speed of light. So we’re going to measure it with a chocolate bar. Let’s get started.



Step 1. You want to stop the platter in your microwave from rotating. This can be done either by inverting the platter or removing the wheels that allow it to spin. Test first.

Step 2. Unwrap the chocolate bar  and place it flat side up on a plate in the microwave. For best results, line up the chocolate with your microwave’s magnetron (that’s the mesh in the wall where the microwaves are emitted).

Step 3. Full power for 15 seconds should suffice. Get the ruler while you wait.

Step 4. Carefully tap a finger across the chocolate’s surface to check for melted points. Check the edges too. If you can’t find any, it’s back in the microwave for another 15 seconds.

Step 5. Once you find melt spots, choose 2 on an axis with the magnetron and measure the spacing between them. Record this length.

Step 6. Time for some math. You might as well eat the chocolate while you’re at it. They say it’s brain food.

Microwave ovens cook food by pumping their molecules full of energy from microwaves, which are really a kind of light. These microwaves form a standing wave that heats food at specific points corresponding to the antinodes of the wave. Without a spinning platter to ensure even heating, these antinodes were melted into the chocolate.

Like all light waves, microwaves have a frequency (microwaves typically use 2.45 GHz) and a wavelength (twice the distance between antinodes, so double the length you measured). The speed of any wave is equal to the product of these two values. Remember to check your units!


v = f (using v for velocity, f for frequency and  for wavelength)

v = 2,450,000,000 Hz x ( 2 x 0.063 m )v = 308,700,000 m/s


The actual speed of light in air is 299,702,547 m/s, and in a vacuum it’s 299,792,458 m/s.


Hopefully you can still taste enough chocolate in your mouth to congratulate yourself on measuring a fundamental universal constant to ±3 per cent in your kitchen and then consuming the results. Truly, science marches on.