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Most microwaves I've used have a plate at the bottom that rotates when the microwave is running. I presume that rotating helps more evenly heat the food.

Interestingly, my microwave rotates by default but has a button you can press to disable the rotation. Under what circumstances or scenarios would you want to stop the microwave from rotating the food?

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up vote 26 down vote accepted

Microwave ovens have a turntable because the microwaves themselves form what are known as 'standing waves'. This means there are essentially static columns of microwave energy inside the cabinet itself.

You can see this for yourself - spread a tray or plate with grated cheese, take the turntable out (or press that button), and zap it for a couple of minutes. You should see bands of melted cheese interleaved with unmelted cheese. Thus, the food is rotated to ensure even heating.

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By measuring the distance between the melted spots of cheese, you can work out the speed of light! –  Skizz Mar 22 '11 at 10:54
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Yes you can. Or you can just remember that it's 299,792,458m/s, using 'We guarantee certainty, clearly referring to this light mnemonic' (the number of letters in the words match the figures) :) This of course the speed of light in a vacuum. Halve it for diamonds! –  ElendilTheTall Mar 22 '11 at 12:03
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@Skizz Actually, since the size of a meter is now defined in terms of the speed of light (that is, the speed of light is defined to be precisely 299,792,458m/s, and the length of a second is defined in terms of the speed of a certain excitation change in Cesium, which means the size of a meter is a function of those two definitions), you can really only use this to either check how much slower light travels in air, or alternatively (if you suck all the air out of your microwave before performing the test), you could use it to check the accuracy of your ruler. –  Theodore Murdock Feb 19 '13 at 19:06
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You stop the turntable when there's not room for your dishes to rotate - think large rectangular dish, or two plates fitting into the corners. At best, the turntable will strain and do nothing, and at worst, it'll make a mess.

Otherwise, it does help to get your food heated evenly. It can't fix everything, of course; the center point is always in the center, not all distances from the center will get quite the same heating, and the middle of your food is always going to take a while to heat up. But it's a lot better than hot and cold spots.

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