You might remember the heroic role that newly-invented radar played in the Second World War. People hailed it then as "Our Miracle Ally". But even in its earliest years, as it was helping win the war, radar proved to be more than an expert enemy locator. Radar technicians, doodling away in their idle moments, found that they could focus a radar beam on a marshmallow and toast it. They also popped popcorn with it. Such was the beginning of microwave cooking. The very same energy that warned the British of the German Luftwaffe invasion and that policemen employ to pinch speeding motorists, is what many of us now have in our kitchens. It's the same as what carries long distance phone calls and cablevision. Hitler's army had its own version of radar, using radio waves. But the trouble with radio waves is that their long wavelength requires a large, cumbersome antenna to focus them into a narrow radar beam. The British showed that microwaves, with their short wavelength, could be focussed ina narrow beam with an antenna many times smaller. This enabled them to make more effective use of radar since an antenna could be carried on aircraft, ships and mobile ground stations. This characteristic of microwaves, the efficiency with which they are concentrated in a narrow beam, is one reason why they can be used in cooking. You can produce a high-powered microwave beam in a small oven, but you can't do the same with radio waves, which are simply too long. Microwaves and their Use The idea of cooking with radiation may seem like a fairly new one, but in fact it reaches back thousands of years. Ever since mastering fire, man has cooked with infrared radiation, a close kin of the microwave. Infrared rays are what give you that warm glow when you put your hand near a room radiator or a hotplate or a campfire. Infrared rays, flowing from the sun and striking the atmosphere, make the Earth warm and habitable. In a conventional gas or electric oven, infrared waves pour off the hot elements or burners and are converted to heat when they strike air inside and the food. Microwaves and infrared rays are related in that both are forms of electromagnetic energy. Both consist of electric and magnetic fields that rise and fall like waves on an ocean. Silently, invisibly and at the speed of light, they travel through space and matter. There are many forms of electromagnetic energy (see diagram). Ordinary light from the sun is one, and the only one you can actually see. X-rays are another. Each kind, moving at a separate wavelength, has a unique effect on any matter it touches. When you lie out in the summer sun, for example, it's the infrared rays that bring warmth, but ultraviolet radiation that tans your skin. If the Earth's protective atmosphere weren't there, intense cosmic radiation from space would kill you. So why do microwaves cook faster than infrared rays? Well, suppose you're roasting a chicken in a radar range. What happens is that when you switch on the microwaves, they're absorbed only by water molecules in the chicken. Water is what chemists call a polar molecule. It has a slightly positive charge at one end and a slightly negative charge at the opposite end. This peculiar orientation provides a sort of handle for the microwaves to grab onto. The microwaves agitate the water molecules billions of times a second, and this rapid movement generates heat and cooks the food. Since microwaves agitate only water molecules, they pass through all other molecules and penetrate deep into the chicken. They reach right inside the food. Ordinary ovens, by contrast, fail to have the same penetrating power because their infrared waves agitate all molecules. Most of the infarred radiation is spent heating the air inside the oven, and any remaining rays are absorbed by the outer layer of the chicken. Food cooks in an ordinary oven as the heat from the air and the outer layer of the food slowly seeps down to the inner layers. In short, oven microwaves cook the outside of the chicken at the same time as they cook the inside. Infrared energy cook from the outside in - a slower process. This explains why preheating is necessary in a conventional oven. The air inside must be lifted to a certain temperature