| Microwaves are like light waves. Microwaves can pass through some things, are absorbed by some things and reflected by other things. Light can pass through clear glass, is absorbed by dark objects (like black upholstery in a car) and is reflected by light objects The unique thing about microwaves, and what makes them useful for cooking, is that they happen to be absorbed by water, fat and sugars. Most foods are comprised of some combination of water, fats and sugars. So, because foods absorb microwaves, they get hot, much like black upholstery absorbs light and gets hot. Microwaves pass through glass and some types of plastic generating little or no heat. The Mechanics of Microwave Microwave oven has in it a magnetron, which is a radio transmitter. If it was on a radio mast (don't try this) it would be able to send radio signals a long way. But it is inside a metal box which keeps the signal in. The frequency of the transmitter is 2450MHz (megahertz), which is a wavelength of 12cm (that's why it's micro waves, rather than short waves (several metres), medium waves (hundreds of metres) or long waves (thousands of metres). There's a good reason for the frequency being 2450 Megahertz, which I'll explain. Food has a high percentage of water, and water is famously H2O. The molecule of water has the O (Oxygen) in the middle, and the two H's (Hydrogen) stuck on it like Mickey Mouse ears at a particular angle (105o). The H's are positive and the O is negative, so the molecule has a + and - end. It has "polarity". Polarised molecules try to line themselves up with the electrical field, like compass needles trying to point at North. But because the electrical field is changing 2,450 million times a second the molecules don't quite have time to line up one way before they have to try to line up the other way! So, anything with water in it has all these molecules being moved this way and that by the electrical field, and heated up. The dishes, walls of the oven, etc, don't pick up radio, so don't get heated up. Other things that pick up radio, anything approximately aerial-like such as the coily filament in a lightbulb, whether working or blown, will be energised if in a microwave oven. If you're silly enough to try any of this, I take no responsibility for what happens. You can take my word for it that the bulb lights up and then explodes. But in contrast, a glass full of petrol (at your own risk be it), which you might assume would explode, will go round and round and not get warmed up at all. That's because the molecules of petrol are long chains of carbon and not POLARISED like the H2O of water molecules. |
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