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 in a 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. 

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, 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. 

Ordinary light from the sun is an example, as well as
X-rays. Each kind, moving at a separate wavelength, has a
unique effect on any matter it touches. When one is exposed
to the summer sun, the infrared rays bring warmth, but
ultraviolet radiation tans the skin. If the Earth's
protective atmosphere weren't there, intense cosmic
radiation from space would be deadly to human beings.
 
So why do microwaves cook faster than infrared rays?
 
Suppose a chicken is being roasted in a radar range. The
microwaves are 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 infrared 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. 

Though weak microwaves exist naturally, scientists didn't
invent devices that harness 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 telecommunications, 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 potential danger they
pose to human health. 

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 unraveled 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?