Radio wave transmittance
Why do radio waves pass through most solids but light does not?
There are two reasons that I am aware of, maybe someone else can
add to it.
1. For a medium or obstacle to block electromagnetic waves, it
must have thickness larger than the wavelength of the waves and it
must have some internal states that respond to energy hc/l, where
h is the Planck's constant, c is the velocity of light and
l is the wavelength of that radiation. It so happens that most atoms respond
to optical wavelengths but the levels corresponding to radio
waves do not exist or are not populated.
2. You can also have free electrons or a plasma which reflects
electromagnetic waves - as the ionosphere reflects waves with
wavelength around 50 metres.
None of these two are satisfied by normal obstacles on
the Earth so . . .
When any radiation enters any material it is partially transmitted
and partially absorbed. As it passes through each layer of the substance a
certain percentage of the radiation is absorbed. How big this "tax" on the
radiation strength is determines how many layers the radiation can make it
through before it "goes broke." Anything if thin enough will be
transparent to any radiation; conversely anything thick enough will be
opaque to any radiation. What determines the tax for radiation of
different frequency is whether there is a place for the energy at that
frequency to go, which is determined by whether there is many processes that
can occur in the material at that frequency. It happens that the vibration
of atoms in all solids is right about at the frequency of infrared light
(heat), so the "tax" for heat radiation is high, and an umbrella blocks
heat radiation from the Sun quite well. The electrons in many solids jump
around at about the frequency of light radiation, so light can also pay a
high tax and many things are hard to see through unless very thin. Radio
waves are at low frequencies, so you need things that happen at low
frequencies (slowly). Rotation of water molecules is one such thing, and
so water absorbs microwave (radio) radiation, which is how microwave ovens
do their thing. Otherwise there is not much, so the tax on radio is light.
Electrons in metals can slosh back and forth in a piece of metal at slow
frequencies, and so metals absorb radio at nearly all frequencies. But the
lowest sloshing frequency is given by the size of the piece of metal, just
as you can get "slower" waves in the tub than in the sink, so to
*efficiently* absorb slow radio radiation like that of AM radio takes long
pieces of metal, of which there are not many just lying around.
Though of course radio waves do get absorbed by some solid objects,
as anyone who has tried to listen to the radio while driving through
a tunnel or behind a hill can testify (it may be blocking rather than
absorbing). Also, there are such things as Faraday cages that are
just big rooms covered with metal that can block most radio from the
interior (the reason is that the metal surface tries to maintain a
constant electric potential, making it hard for a fluctuating field
to get through).
The fundamental reason for the different behavior of different materials
has to do with the way the electron quantum states in the solid
are filled up. The electrons go in and fill all the states up to
some energy, until there are no more electrons left. If the next
state available for a new electron is at essentially the same energy as the
last electron that went in (as is true for all metals) then the
electrons behave as if they were "free" and block electric fields.
If there is a "gap" in energy to the next available states then the
electrons are not so free - they can only absorb, reflect, or otherwise
tamper with electromagnetic radiation if it is at a frequency that
is equivalent (using Planck's formula) to this energy gap. This
is what leads to the different coloring of different materials - they
all have different energy gaps. Metals are special, because they
have no energy gap at all. Semiconductors are kind of in the middle -their
gaps are generally pretty small, but usually too high to block
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Update: June 2012