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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?