Edwin wrote to me:
"I read in the Times today that you can work out the speed of light using a microwave and a bar of chocolate! (apparently microwaves travel the same speed as light.)"
And poor Edwin got a physics lesson back from me:
Microwaves are the same as light, just different frequency:
It goes like this, in increasing frequency (same as increasing energy -
the ones with more energy are more "dangerous" or harmful):
(image copied from NASAexplores)
infrared (the heat that comes off a fire, feel it if you put your hand to the side of eg a candle - above a candle you can feel the hot air rising as well - to the side only the infrared "light")
visible light (normal sun light)
ultra-violet (what gives you sunburn)
These all just different sorts of "light", so travel at the speed of "light". As the frequency goes up (energy goes up) that just means there are more waves going past you (per minute), like peaks of waves on the sea going past you. So if there are more "peaks" going past you, the peaks have to get closer together.
The distance between the peaks (the wavelength) is what they were measuring in the chocolate in the microwave. For any wave there is a formula which gives you the speed if you've got the wavelength (distance between the peaks) and the frequency. The frequency of a microwave oven is printed on the oven (on the edge of the door in mine).
Actually, a bit better explained on this NASA page: Measuring the electromagnetic spectrum.