If you’ve ever made brownies or chocolate cake, you might have had to melt chocolate in a standard 2450 MHz microwave oven. Since all electromagnetic waves travel at the same speed, finding the speed of a microwave is the same as finding the speed of visible light.

First, remove the turntable and wheels from the microwave. Unwrap a solid chocolate bar (it must be one without any fillings or toppings) and place it in the centre of the microwave on a plate like below.

Then set the microwave to run for around 20 seconds. You should see atleast 2 spots where the chocolate has significantly melted, and other spots where the chocolate is still solid. If the chocolate bar is completely unmelted, microwave it for a further 10 or 20 seconds. If the chocolate bar has instead completely liquified, allow it to solidify in the freezer before reattempting.

The reason this non-uniform melting occurs is because stationary waves (sometimes called standing waves) appear in the oven as microwaves reflect from inside the walls and interfere with the incident waves. Where the waves meet in phase (peaks meet peaks or troughs meet troughs), the amplitude of the resultant wave is higher, and thus the energy transferred by the wave at this point is also higher, resulting in more melting. These “hotspots” are half of a wavelength apart from each other (as troughs and peeks are also half a wavelength from each other on a single wave).

Using a ruler, measure the distance between the centres of 2 hotspots. This distance is half of a wavelength of light, so multiply it by 2.

In my experiment, this distance between hotspots (marked with purple lines) was around 6cm (around 2.4 inches in freedom units).

If 6cm is half a wavelength, then 12cm or 0.12m is a full wavelength.

Your microwave will tell you which frequency it uses, which is generally always 2450 MHz or 2.45×10^9Hz.

The speed of an electromagnetic wave is equal to the frequency times the wavelength, and so the speed of light, as measured in my experiment was:

(2.45×10^) x (0.12)=2.94×10^8 m/s

The true value is actually 3×10^8 m/s, so we are close.

This is a common classroom practical to demonstrate both the wavespeed equation and the properties of stationary waves.