Today I brought in some leftover fish pie for my lunch. I heated it up in our office microwave, but didn't notice that the turntable had slipped off its axis, so the whole thing didn't turn. When I came to eat the pie, there were hot and cold patches on the top, in a classic interference pattern. Microwaves always have these hot and cold areas, which is why the turntable is important.
Sadly the pie disappeared before I could take a picture
My tupperware box is about 20cm across, and the hot patches were about half its width across, so I estimated the distance as 10cm. The label on the microwave gave the frequency of the magnetron as 2.45GHz. The relationship between wavelength and frequency of a wave is as follows:
wavelength = speed of wave × frequency
or:
speed of wave = wavelength ÷ frequency