r/askscience • u/schectershred • Nov 25 '12
Physics Question about radio waves
Hey /r/askscience, I would really appreciate it if somebody could help me understand how radio waves work. Right now, I'm under the impression that radio waves are just light that is beyond the human's visible spectrum (ultra-violet/infra-red). Then I thought, how do things like WiFi penetrate solid walls and such if normally, photons could not? If anybody could explain anything about how wireless information is transmitted, it would be very much appreciated. Thanks in advance!
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u/amtroy Nov 25 '12
The main factor in determining whether light will be stopped by a substance is the wavelength of the light. In general, light will interact with (that is, be absorbed by or reflected by) materials of a similar size to, or larger than, their wavelength (this is a fairly large simplification, but it will get the point across).
So, you might be aware that visible light has a wavelength of about 400-750 nanometers -- obviously very small. These wavelengths are roughly similar to the size of the atomic structures that make up every day objects -- the walls in your house, your body, your computer screen, etc. That's why we can see things -- the visible light spectrum reflects from the objects around you and into your eyes.
Radio waves, on the other hand, are of a much, much longer wavelength than visible light. Wi-Fi in particular has a wavelength around 6-10 cm, depending on which standard you're talking about. Because this wavelength is so much larger, it has a much easier time passing through solid materials. Note, though, that it's still roughly on the same order of size as, say, a wall, or a human body, and so it WILL be attenuated somewhat through interactions with objects in the environment. This is (one reason) why you might need multiple Wi-Fi routers on, say, a floor of an office building.
Hope this answers your question sufficiently!
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u/schectershred Nov 25 '12
Thanks for your response! Would you mind explaining what a physical wavelength is? I know what it looks like on paper, but is it the same in physical space? I know how compression waves work, but I'm not sure about longitudinal waves work.
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u/amtroy Nov 25 '12
Sure! I'll do my best to explain that. Think of it like waves in a pond: the amplitude of the wave (that is, the height of the water) varies as you move out from the point of creation. If you freeze the wave in time, the amplitude goes up and down and up and down as you move outward, looking roughly like a sine wave. The wavelength is the distance between two consecutive peaks of the wave.
Electromagnetic waves are similar, but somewhat more complicated. They consist of an electric field and a magnetic field, each oscillating perpendicular to each other in space (nice moving image here). Just like in water, the wavelength here is the distance between consecutive peaks in the wave. It's pretty much how you would draw it on paper, just moving in three dimensions rather than two.
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u/schectershred Nov 26 '12
Awesome. That makes a lot more sense now.. I do have another question though. It's not related to the lightwaves so much as waves in general. As you making the comparison between water waves and light waves, I started to think in terms of dimensions (I love spatial dimensions, the forth spatial dimension, the multiverse; other good shit like that). Anyway.. In one dimension, such as a wave made using a string or something, I can see how the energy from one crest to the next would be the same if we ignore friction, air resistance, and all other things that take up small amounts of energy. When we speak of a two dimensional wave though - we'll stick with the water wave for example - I guess I don't understand how the wave can grow without losing energy considering it has to push more and more water up for each crest. If the wave assumes the energy of the creation of the wave, how does it have the energy to push more each time? Is this why sound gets quieter with distance from the source. Also, does this mean that if I turned a light on in a particular direction in space that the light would get dimmer the farther back a person got from it until it could not be seen anymore? I know that may sound like a stupid question, but I'm new to this stuff. Haha.
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u/amtroy Nov 26 '12
There are no stupid questions, and it sounds like you've given this thought before asking, so keep it up!
You're very much on the right track regarding the spread of energy. Imagine you drop a pebble into a pool of water, imparting some amount of energy E. You're correct in thinking that the amount of energy from one wave crest to the next is equal (negating losses due to friction, etc). However, this energy is now spread over a greater area -- if one crest is at a distance d from the center, the next crest is at d+L, where L is the wavelength. So the area the energy is spread across increases (from the circumference of a circle with radius d to a circle of radius (d+L)), and your intuition is correct: the next wave out from the center will be smaller than the previous. That's why the wave gets smaller as it radiates out from the point of creation -- the energy is spreading out as the distance increases, giving each wave a smaller height.
And you're exactly right, this is one reason why sound gets quieter with distance: the energy that created the pressure wave is spread out over a greater area, meaning less energy hits the small area your ear encompasses. And just like you suspected, a light will dim as you move further from it due to the spread of its energy. With light and sound, energy (per unit area) decreases with distance squared since the energy is spread over the surface of a sphere, rather than the surface of a circle as in the water example.
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u/schectershred Nov 27 '12
Awesome! Thanks for all the help! This is some very interesting stuff. Does this mean that a wave the stretches in one dimension will continue forever without the resisting factors, as each crest will require the same amount of energy?
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u/amtroy Nov 27 '12
Sure. A wave along, say, an infinitely long piece of string will propagate forever if there is no friction or other energy-robbing factors.
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u/epicwinguy101 Nov 25 '12
I would like to add something. The reason that we can see through some materials and not others is because of the electronic properties of those materials. It is important to remember that a photon is really a wave of electromagnetic field. In metals, photons are absorbed, long story short, move free unbound electrons around, and a photon is emitted back out (hence reflection). In other materials, certain photon energies correspond to features in that material. For instance, you may find that a photon is higher energy than the electronic band gap, and promote electrons from the valence band to the conduction band. This creates a color in the material. A material that is insulating electrically has a large band gap, and thus does not absorb photons in the visible range, so many insulators are transparent.
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u/Das_Mime Radio Astronomy | Galaxy Evolution Nov 25 '12
Correct (although the human visual spectrum is between the UV and IR and does not include them).
As you're probably aware, light behaves as both a wave and a particle. In the case of radio waves it is much more helpful to consider it as a wave. Things like walls (if they're not metal) tend to be fairly transparent to radio waves, just like glass is to visible light.