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Microwaves
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 by the infrared rays
before it can heat the food properly..
It also explains why infrared ovens brown food and microwave ovens
don't. Bread turns crusty and chicken crispy in a infrared oven simply
because their outside gets much hotter than their interior.
Finally, as anyone who owns a microwave oven knows, you never put an
empty container inside a radar range. Since nonpolar materials such as
plastic and glass don't warm up in the presence of microwaves, there will
be nothing in the oven to absorb the radiation. Instead, it will bounce
back and forth against the walls of the oven, creating an electrical arc
that may burn a hole in the oven.
This hushed energy, electromagnetic radiation, flows all around us.
All forms of matter, even your own body, produce electromagnetism --
microwaves, x-rays, untraviolet rays.
It may interest you to know that whereas the human eye is sensitive to
light radiation, the eye of the snake can sense infrared. Your body emits
infrared radiation day and night, so snakes can see you even when you
can't see them.
Though weak microwaves exist naturally, scientists didn't invent
devices that harnass them for useful purposes until the 1930s. In a radar
range, the device from which microwaves emanate is a small vacuum tube,
called a magnetron.
A magnetron takes electrical energy from an ordinary household outlet
and uses it to push electrons in its core so that they oscillate fast
enough to give off microwaves. These are then relayed by a small antenna
to a hollow tube, called a waveguide, which channels the microwaves to a
fanlike stirrer that scatters them around the oven's interior. They bounce
off the oven walls and are absorbed by water molecules in the food.
The U.S. Environmental Protection Agency estimates that our exposure to
electromagnetic radiation increases by several percent a year. Look
around you. The modern landscape fairly bristles with microwave dishes
and antennae. Here again, in telecommuncations, it is the convenience with
which microwaves can be focused in a narrow beam, that makes them so
useful. Microwave dishes can be hundreds of times smaller than radio wave
dishes.
Industry employs microwaves heat in many ways -- to dry paints, bond
plywood, roast coffee beans, kill weeds and insects, and cure rubber.
Microwaves trigger garage door openers and burglar alarms. The new
cellular car phone is a microwave instrument.
Microwaves and Your Body
Not surprisingly, as high-powered microwaves have proliferated in the
atmosphere and the workplace, a passionate debate has grown over the
pontential danger they pose to human health. But that is a topic for
another article.
For the moment, scientists at the University of Guelph have recently
reported using microwaves to raise chickens. Housed in a large oven-like
enclosure, young chicks keep warm under a slow drizzle of radiation. So
far, the chicks seem to like their home in the range. They've even learned
to turn on the microwaves whenever they feel cold.
A similar scheme for heating human beings has actually been proposed
by a scientist from Harvard University. Equipping buildings with
microwave radiators would cut energy costs, he says, since microwaves heat
people and not the surrounding air.
Just set the thermostat dial to rare, medium or well done! Some
researchers are concerned that people who work with microwave equipment
are absorbing low levels of radiation that may prove harmful over the long
term. One line of experiments has shown that uncoiled DNA molecules in a
test tube can absorb microwave energy. The unravelled DNA chains resonate
to the microwaves in the same way that a violin string vibrates when
plucked. The question this raises is this: does microwave radiation
vibrate coiled DNA in the human body, and if so, is this vibration strong
enough to knock off vital molecules from the chain?
ADDITIONAL FEATURED ESSAYS
Microwaves
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Microwaves
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