radio waves

December 29, 2007 12:17 PM

How do radio waves travel through walls?

I assume the waves penetrate a solid at the microscopic level, causing them to vibrate at the frequency of the wave which then travels into the enclosed room, but I really have no idea.

posted by amsterdam63 to Technology (14 answers total)

nope, they just go right through. its not sound, its an electromagnetic wave. either the material absorbs photons or it doesnt.

check out feynman's book QED which deals with this topic ("the strange theory of light and matter")
posted by joeblough at 12:24 PM on December 29, 2007

This may help explain it. Basically if electromagnetic waves (including radio waves and light) can be absorbed, reflected or penetrate different materials, this depends on the frequency of the wave and the atomic structure of the material. Just as air and glass are transparent to light waves, many materials are more or less transparent to certain radio frequencies.
posted by TedW at 12:27 PM on December 29, 2007 [1 favorite]

In other words, some materials are transparent to visible light, like glass. Some are opaque, like wood. Some are transparent to UV light, but not to visible light, like a thin t-shirt that leaves you with a sunburn. Some are transparent to visible light but not to UV, like the material used for sunglasses.

Same thing applies to radio waves: many materials are transparent at those frequencies, even if they aren't at other frequencies (like visible light).
posted by alexei at 12:29 PM on December 29, 2007 [3 favorites]

yeah - i should have said "absorbs photons of a particular frequency/energy or it doesnt"
posted by joeblough at 12:31 PM on December 29, 2007

To gain some understanding by analogy: radio waves are EM, like visible light. At a gross level, an EM wave hitting an object is "used up" in three ways: Reflection, absorption, and transmission. You'll note that things like windows and water, when hit by visible light, transmit most of it - they are transparent to visible light. This response is selective in wavelength, e.g. a window tinted so that only red light is transmitted. Walls are transparent to the wavelengths of radio waves.
posted by TheOnlyCoolTim at 12:34 PM on December 29, 2007

And to add to this, not all walls are transparent to radio. In particular, concrete and rebar construction is very good at absorbing VHF, UHF and Microwave frequencies. However, for most wood-frame residential construction there is very little in the wall that absorbs radio.
posted by KirkJobSluder at 1:02 PM on December 29, 2007

Obligatory Wikipedia link: the extinction coefficient.
posted by you're a kitty! at 1:22 PM on December 29, 2007

Windows are transparent in part because we choose to make them out of a material that is transparent at the frequency of visible light.

For radio waves, it is the other way around. They go through walls because we have chosen for radio a frequency band at which most building materials are transparent.

As others have mentioned, if you would like to know why some materials are transparent, or why they have the colour they do, you will need to read some physics and some chemistry.
posted by gmarceau at 1:42 PM on December 29, 2007

And to add to this, not all walls are transparent to radio. In particular, concrete and rebar construction is very good at absorbing VHF, UHF and Microwave frequencies. However, for most wood-frame residential construction there is very little in the wall that absorbs radio.

(Hopefully not a tangent): So does this explain why you sometimes (especially with AM radio) lose the station for a second when you drive under an undepass?
posted by 4ster at 2:02 PM on December 29, 2007

4ster: Yeah, that's a great example, although AM does better than a lot of the higher frequencies which are just slightly better than line of sight in many urban areas.
posted by KirkJobSluder at 2:14 PM on December 29, 2007

you might as well ask how they penetrate air
posted by caddis at 5:54 PM on December 29, 2007

See also Faraday Cage for how it stops EM waves from propegating.
posted by jeffamaphone at 6:23 PM on December 29, 2007

With respect, I believe the questioner's explanation is closer to the mark than many of the answers here. Take visible light passing through glass. My understanding is that it is absorbed by atoms in the glass, bounces around the electron shells and, being of an unsuitable quantum, is spat out the other side.

You know this because all of that takes a little bit of time, as a result, the light slows down. Thus we can make eyeglasses and telescopes. If light just went straight through, this wouldn't be possible.

You might indeed ask how radio waves penetrate air. They do exactly the same thing. In certain atmospheric conditions - called an inversion - your radio will suddenly pick up stations from different states or countries. The air is acting like a lens, bending the radio waves in space. Again, this wouldn't be possible if the waves just went straight through.

Nothing about light is simple. Asking questions like the one above and answering them carefully and honestly is what created Physics.
posted by grahamwell at 4:27 AM on December 30, 2007

So does this explain why you sometimes (especially with AM radio) lose the station for a second when you drive under an undepass?

Yes, but there's something additional going on too. After all, the radio waves can enter the underpass from either side, since both sides are open to the air. Shorter radio waves, llike FM, can fit into the underpass better than longer radio waves, like AM. And if the underpass is small enough, or long enough, radio waves have a real hard time making it to your antenna, and you'll lose the signal.
posted by exphysicist345 at 5:31 PM on December 30, 2007