rf - Free and Open Source Software

to an excitation from the incoming wave.
If the incoming wave is at a frequency far
below that of the particle's resonance, the
particle will vibrate weakly in phase with the
• •
mcormng wave.
If the incoming wave is at a frequency far
above that of the particle's resonance, the
particle will still. vibrate weakly at the
frequency of the incoming wave, but its
vibration will be 180 0
out of phase with the
incoming excitation.
If the incoming wave's frequency matches
that at which the particle is resonant, the
particle will vibrate strongly, 90 0
out of
phase with the incoming wave.
In most materials the particle resonances
are at frequencies higher than that of visible
light; a few substances have resonances as
low as the infrared region, but almost none
have resonance in the common rf range.
For this reason, for most rf energy and
almost all materials the first case will hold
true. Each particle at the boundary of the
material will vibrate weakly and in phase
with the incoming wave.
There's a very special exception which we
will meet a little later, in which both
in-phase and out-of-phase vibrations occur.
Before we look at that, though, let's stay
with the first case and see what happens
most of the time.
Now as it happens, a vibrating particle
will itself emit new radiation just because it's
vibrating. It's the same basic idea as that of
the tuning fork, which you hit to make
vibrate, and which then emits an audio wave
because it is vibrating.
This means that when an r[wave hits the
surface of any substance, each particle at the
surface of that substance will re-radiate new
waves which are in phase with the original
rf
Each of these new waves will, in turn, hit
adjacent atoms or particles within the material
and cause additional vibrations and more
re-radiation,
If the particles are scattered about the
substance more or less at random, as they
are for instance in a gas, the total effect of
all this secondary vibration will be a "scattering"
or "diffusion" of the original wave.
The higher-frequency waves in the original
energy (if a mixture of frequencies were
present at the start) will predominate in the
scattered new radiation, because they had
more energy per photon to begin with.
We see such an effect any time we look at
a blue sky. The blue skylight is the scattered
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