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Q & A: very low frequency radiation

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Most recent answer: 04/22/2013
Q:
Why does wave in low frequency (long wavelength) can penetrate water and can follow the earth's surface?
- Hai-Binh LE (age 31)
Hanoi, Vietnam
A:
Hai-Binh-

The main way that water affects very long wavelength radiation is by its electrical conductivity, especially due to any salts dissolved in it. The very long-wavelength radiation penetrates water farther than short wavelength radiation for the same reason that it penetrates any conductor farther. The typical penetration depth, called the electromagnetic skin depth, scales as the square root of the free-space wavelength. Perhaps rather than include the explanation here, we should just link to a site that has a nice account of the math:
. Please follow-up if you'd like further explanation of that account.

Very low frequency waves follow the Earth's surface because they are trapped between the surface and the ionosphere. () Both the Earth's surface and the ionosphere are fairly good electrical conductors, so the waves bounce of them a bit like the way light waves bounce off metal. The ionosphere conducts because ultraviolet rays from the sun tear apart some air molecules, leaving electrically charged particles (ions).

Mike W.




(published on 04/22/2013)

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