Undated post, migrated from old site.
I got this off a TV programme the other day. It’s one of those things that seem so blindingly obvious, once someone’s pointed them out, that you wonder why you never thought of it. All you need is a microwave oven, some cheese slices, or anything else that’s easily meltable, and a metric ruler. And some children, to give you an excuse to try it under the guise of teaching them some physics.
First take the turntable out of your microwave. It’s there to make sure that anything you put in there is heated evenly, and that’s the last thing you want for this experiment. If the turntable is turned by a mechanism that sticks up proud of the floor of the oven, you’ll want to use something to raise whatever you put your cheese on clear of it.
Now take a microwave-safe tray or a large plate, and cover it with your cheese-slices (or whatever), pop it into the oven and start it cooking. I can’t tell you how long for, as different thicknesses of slice, types of cheese, and so on, will render my figures useless. What you want is for the cheese to heat up just enough for hot-spots to show. (Those of you who remember when microwaves didn’t have turntables will know exactly what I mean.) If you’re not sure, check it every half-minute or so, but don’t move it while doing so.
As soon as hot-spots appear, take it out of the oven and, as accurately as you can, measure the distance between the melted bits, centre to centre. Double that figure, divide it by whatever it takes to convert it to metres (100 if you measured in cm, 1000 if you measured in mm. So 6cm would give you (2 × 6) ÷ 100 = 0.12), and jot it down. That’s given you the wavelength in metres.
Now take a look at the back of the microwave. You’re looking for the frequency. It’ll most likely be given in megahertz (MHz). A quick google tells me the normal standard is 2450 MHz, in the UK at least, but you’re better off checking your own if you can. You’ll want the figure in hertz, so multiply by 1,000,000 and jot that down too.
The speed of a wave in metres per second = the frequency in hertz × the wavelength in metres, or, c = f × λ
Plugging in the figures I used as an example;
C = 2,450,000,000 × 0.12 = 294,000,000 m/s
The actual value of the speed of light (through a vacuum) is 299,792,458 m/s, so not a bad result for an experiment you can do with some cheese and an ordinary ruler.
How close can you (or your kids) get? Maybe experimenting with different foods, looking for one that shows the hot-spots more precisely, would get you closer still. And any failures will still be edible…
Happy cooking,
—Daz
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