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Why can radio waves go where light cannot?
Name: Observe
Status: N/a
Age: N/A
Location: N/A
Country: N/A
Date: Around 1993
Question:
Visible light is more energetic than electromagnetic radiation in
the radio part of the spectrum (say, 1 MgH). Therefore, visible light should
be able to penetrate to where radio waves cannot. And yet, the reverse seems
to be the case - I can keep out light with plastic or wood, even though less
energetic radio waves have not problem penetrating these materials. Why is
that?
Replies:
The interaction of EM radiation with matter is really a complex
process. There is an oscillating electric field associated with an EM wave;
the electrons (and to some extent the protons) in the material on which the
radiation falls move in response to that electric field. An electron bound to
an atom, or atoms bound together in a molecule, act a bit like a mass attached
to a spring. If you wiggle the "free" end of the spring periodically, the
mass will oscillate. If the wiggle rate is near the "natural" frequency of
the spring, the mass will strongly respond (this is called resonance);
however, if the wiggle rate is much less or much more than the "natural"
frequency, the mass will not respond much. Depending on the nature of the
electronic or interatomic bonds, a given frequency of radiation may be
transmitted, absorbed, or reflected. So radio waves penetrate most substances
because their frequency is too low to excite the electrons or atoms. Metals
reflect them because their outer electrons are virtually free to move around,
so they respond almost instantly and reflect the wave. There is a limit to
this; for high enough frequency, these "free" electrons cannot keep up with
the electric field: most metals transmit ultraviolet light. And in general,
for high enough frequency, the electrons and atoms of most materials respond
weakly. Since a photon's energy is proportional to its frequency, it is true
that high-energy stuff penetrates matter more readily, but it is because of
its being high-frequency, not because of the energy.
R.C. Winther
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