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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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