The theory behind it is ...
Microwaves (and light) are energy waves. The waveform repeats along its length at intervals, each known as a cycle. The cycle has a wavelength. Each wave cycle has two points of high energy and two of low energy.
The points of high energy are where the margarine will melt first.
So measuring the average gap between the centres of adjacent patches gives us a distance that is half the wavelength.
The wavelength is a distance in metres, the length of one cycle.
The frequency is the number of those cycles per second.
Multiplying them together gives a figure that has units of metres per second. That means it is a measurement of speed.
Taken from
http://www.bbc.co.uk/bang/handson/lightspeed.shtml
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