From: philip@mentorg.com (Philip Peake)
Subject: Re: Quick radar question about traffic ticket
Date: Fri, 06 Nov 1992 19:13:46 GMT
Organization: Mentor Graphics
Keywords: 
Followup-To: 

The following doesn't have too much to do with autos ... but you
may fond it interesting ...

In article <Bx9EMo.2F9@mentor.cc.purdue.edu>, mshar@mentor.cc.purdue.edu (Mike Mshar) writes:
|> 
|> OK, then, what is the amplitude of a radar gun`s emmited radiation? I
|> understand the microwave part, i.e. the holes in the screening are smaller
|> than the amplitude of the waves, therefore the waves make contact with the
|> metal screen as they pass through, and are therefore absorbed. But I was
|> under the impression that radar waves were not very powerful, and did not
|> have a large amplitude. So, my reasoning was that they would be able to
|> pass through a radiator. Besides, doesn`t the clear plastic door panel of a
|> microwave oven reduce the energy of the exiting waves to the point where
|> they are not harmful to humans? I know that when your car is parked in the
|> sun, UV waves go through the glass into the car. On their way through the
|> glass, some of their energy is lost, and thus they are reduced to infrared
|> waves. When this happens, the waves no longer have enough energy to pass
|> back through the glass, so they just bounce around and are absorbed inside
|> the car, thus producing the heat. Doesn`t a microwave oven door do the same
|> thing to waves coming out of it??

You seem slightly confused on a couple of issues. This is porobably because you
are trying to map ME knowlege onto Electro-Magnetic RF stuff ...

To try to get a better picture, stop thinking of RF as the sine wave that
you see in diagrams in text books.

The important thing in trying to understand what happens is not the AMPLITUDE,
but the FREQUENCY, and hence the WAVELENGTH of the emission.

If the holes in a screen are smaller than the WAVELENGTH, then the radiation
will not pass through.

The AMPLITUDE (or POWER) of the RF has no effect on this.

You are also mixing up another physical phenomeon when talking about the car
and windows (its also the same principal as global warming).
Basically, imagine it as follows:

As you inject energy into an object, its temperature will increase.
Nature always attempts to equalise things, so having one thing at a
higher temperature than its surroundings is un-natural - the hotter object
will attempt to shed its excess heat (energy).
If we assume that this is taking place in a vaccuum, it is east to see that the
only possible way of getting rid of the energy is by radiating it.


As you pump more energy into this object, it has to get rid of energy faster,
and does this by radiating more energetically - the wavelength, or frequency
of the radiation increases - this is why as you heat a lump of metal, it begins
glow red, then orange, then yellow ... etc the frequency of the radiated
energy increases.

Now, back to cars ... the energy from the sun arrives at fairly high energy
levels - these energy levels can penetrate glass. They afre absorbed by the
car's interior, and so the energy level of these objects increases, and they
attempt to shed the excess energy by radiating it. Unfortunately (well,
maybe fortunately :-) the interior objects of your car are not at the same
energy level as the sun, and so radiate the energy at much lower
frequencies. At these lower frequencies, glass in not transparent, and so the
energy cannot pass by radiation. It is hence re-absorbed, byt other parts of the
car and by the air inside it - energy is being pumped in and absorbed, but
cannot escape - at least not until everything warms up to the point
where the radiation being produced is high enough in frequency to be able
to pass through the glass once again.

I hope this rather simplified description helps ...

Philip
Organization: Senior, IM - H&SS Track, Carnegie Mellon, Pittsburgh, PA