Updated: Aug 5, 2021
Our Chief Assessor, Toby, explains one lesson where he was taught how to measure the speed of light using a bar of chocolate!
One of the most memorable interview lessons I’ve ever had was a Physics lesson in which I was taught how to measure the speed of light using only ordinary household items. What’s that, you say Toby, measure the speed of light at home?! I was skeptical too. Light moves much too quickly to measure its speed by using a stopwatch to see how long it takes to cover a certain distance. It can be done, however, using a rather ingenious method that is in fact quite simple.
All you need is a microwave oven, a ruler, and a bar of chocolate. As my tutor explained to me, a few pieces of information need to be established before the experiment can be done.
1. Firstly, we need to understand that visual light is a wave and part of the electromagnetic spectrum. So are microwaves. All electromagnetic waves travel at the same speed. So if we can measure the speed of a microwave, we can by implication measure the speed of light.
2. Waves have a few different properties. These include wavelength, amplitude, speed, and frequency. The amplitude is unimportant for the lesson at hand. The wavelength is what it sounds like: the length of one complete wave, or the measurement between two adjacent peaks. The frequency is the number of waves completed per second.
3. Crucially, three of the above properties are linked by the following equation
Speed = Frequency x Wavelength
This means if we know the wavelength and we know the frequency, we can calculate the speed.
4. The way a microwave oven works is by creating standing waves that create constructive and destructive interference with each other. This means at some points in the oven, which can be visualised as the peaks and troughs of a sine wave, there is a great amount of heat and activity. At other points, due to destructive interference, there is no activity at all. These are the points halfway between a peak and a trough.
This is why microwave ovens have those rotating discs and also why instructions on microwave cooking often advise you to take the dish out and stir it halfway through. These measures are designed to ensure the food is heated evenly throughout, rather than very hot in some places, and stone cold in others!
Right then, having established the above four points, we’re ready to measure the speed of light! The first step is to look at the microwave oven to check the operating frequency. This should be recorded somewhere on the body of the oven, and is a number measured in megahertz, or MHz. Next, we need to take out the rotating disc. In order to measure the wavelength, we will need the chocolate bar to remain stationary as it is being heated. Unwrap the chocolate and place it in the oven. Then close the door, and start heating! What should happen (fairly quickly) is that the chocolate bar will begin to heat in two distinct places.
These are the two peaks of the standing wave, the “hot spots” mentioned earlier. Once these hotspots have appeared, all you have left to do is take the chocolate out and use your ruler to measure the space between the two wave peaks in order to find the wavelength. Multiply the wavelength by the frequency and voila, there we have it, the speed of light!
Why was this such a great lesson? Several reasons. First of all, it was nicely self-contained. The student does not need extensive physics knowledge in order to grasp the points above, and gets to reach a satisfying conclusion quickly. Secondly, rather than simply teaching abstract theory, the lesson contained an observable real-life experiment to make the concepts more concrete. And finally, the idea of measuring the speed of light is very exciting, and bound to capture the imagination of even a reluctant GCSE student.
Blog Post Crafted by Toby
Toby is in charge of recruitment of new tutors. He conducts interviews with prospective tutors and assesses their lessons to get a feel for whether they have the teaching style we're looking for.