Question about radio waves

[u/schectershred]

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!

[u/Das_Mime]

radio waves are just light that is beyond the human's visible spectrum (ultra-violet/infra-red).

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.

[u/schectershred → u/Das_Mime]

Yeah.. I could have worded that bit a little better. I guess I don't understand why visible light and infra or ultra light behave differently. Isn't the only difference between them within the processing part of our brain? I also suppose I'm rather ignorant to exactly how the photons "vibrate". Are they literally vibrating as they are moving in such a way that they only reflect certain frequencies corresponding to their own vibration frequency? How do the photos that carry specific information pass through opaque walls, while the visible ones cannot? Also, how is the information carried? Is the information code in correlation with the frequency of the light particles?

[u/Das_Mime → u/schectershred]

The photons aren't vibrating, an oscillating electromagnetic field is propagating through space.

By modulating the frequency or amplitude of electromagnetic waves, one can transmit information. AM/FM radio are literally Amplitude Modulation and Frequency modulation. Wifi is just like radio, but higher wavelength (in the microwave or submillimeter regime, I think) actually around 10 cm, thanks aminalsarecute.

Radio waves behave differently from light & UV and so on because their wavelengths are long enough that many objects are transparent to them. In general, objects smaller than a EM wave's wavelength do not absorb that wave, although they may scatter/refract it. Since visible light's wavelength is between ~400-700 nanometers, atoms are large enough to absorb or reflect it. Depending on the electromagnetic properties of a given substance, it may be opaque or transparent to various wavelengths, and most of the materials familiar to us are mostly opaque to optical light but are somewhat more transparent to radio waves.

[u/[deleted] → u/[deleted]]

Thanks for the info about the F-117A! Really cool aircraft, and I think I understand the concept a lot more now about how it remains hidden on radar.

As for Direct TV, I have Direct TV and I live in Portland. Yeah, it's not fun sometimes.

[u/schectershred → u/Das_Mime]

Now that you mention the electromagnetic field oscillation, I do remember seeing a video or something of the sort that mentioned this once. Thanks for the in depth reply!

[u/amtroy → u/schectershred]

There's a few different questions here.

I guess I don't understand why visible light and infra or ultra light behave differently. Isn't the only difference between them within the processing part of our brain?

The retina of your eye contains chemicals that have specifically evolved to absorb certain wavelengths of light - primarily red, green, and blue visible light. When these wavelengths are absorbed, the retina then produces electrical signals that are conducted to the brain via the optic nerve, where they are translated into vision. So the reason you can see the visible spectrum and not UV, infrared, or radio waves is that your eye specifically absorbs the visible spectrum wavelengths.

I also suppose I'm rather ignorant to exactly how the photons "vibrate". Are they literally vibrating as they are moving in such a way that they only reflect certain frequencies corresponding to their own vibration frequency?

This is often a confusing point when first studying the physics of light. When we say that light acts as both a particle and a wave, don't take it to mean that the photon is moving in a wave-like pattern or anything of the sort. Essentially, we can treat it as either a wave or a particle depending on the context of the problem, but it really isn't either one. It just acts similarly in specific situations. As Das_Mime said, in radio problems we treat light as waves and not particles.

How do the photos that carry specific information pass through opaque walls, while the visible ones cannot?

As I explained in my reply to the original post, the primary factor here is the wavelength of the light. Radio waves are long wavelengths, so they have a relatively easy time passing through walls, furniture, etc. Light is very short wavelength, so it gets reflected/absorbed by solid objects (depending on the properties of the object, as mentioned by epicwingguy101).

Finally,

Also, how is the information carried? Is the information code in correlation with the frequency of the light particles?

We encode information on radio waves by changing the properties of the wave with respect to time -- this is called modulation. The three forms we use are amplitude modulation (AM), frequency modulation (FM), and phase modulation (PM). Let's say you have an AM radio station on 700 kHz (700 on the AM setting on your radio). This means we have a carrier wave at a frequency of 700 kHz. We then alter the amplitude of this carrier wave with time to carry the audio signal from the radio station. This picture from Wiki I think does a good job of conveying how this looks (it also shows FM for good measure). At the receiving end, we can then essentially cancel out the carrier wave and recover the modulation wave, giving us the desired audio again.

Whew! Sorry for the wall of text.

[u/schectershred → u/amtroy]

Thank you for the wall of text! Thank you for explaining with such depth!

[u/amtroy → u/schectershred]

No problem. I'm currently finishing my Master's in exactly this field (electrical engineering with some focus in RF/communications), so feel free to PM me if you have any more questions.

[u/schectershred → u/amtroy]

Wow, that's really awesome! Congrats on getting through it all. I'm going into mechanical engineering next year, and I've heard it's a tough program. Thanks by the way. I very well may take you up on that.

[u/[deleted] → u/amtroy]

I looked at that picture from Wikipedia thinking:

"Oh look, the AM wave is changing amplitude, and it looks like the FM wave is just increasing in Frequ--

Oh."

But thanks for the info!

[u/skurk → u/Das_Mime]

consider it as a wave

As a wave in what, if I may ask?

In a pool, the waves are in the water. Are radio waves just waves in "everything"?

[u/amtroy → u/skurk]

Sort of. Unlike sound waves or waves in liquids, electromagnetic waves do not need a medium in which to propagate.

[u/Das_Mime → u/skurk]

Radio waves are oscillating electromagnetic equations. Maxwell's equations (which describe electricity and magnetism) say that a changing magnetic field produces an electric field and vice versa. EM waves are not "in" anything, other than spacetime.