61
votes
Accepted
What is the scientific explanation for radio waves bending around the Earth?
EDIT: In the interest of avoiding spreading misleading information, I have removed the portions of this answer that have been disputed or refuted in the comments and edits on this question. ...
53
votes
What's the physics behind XKCD #2027 (time between lightning flash and radio wave burst)?
I think it's fair to say that explainxkcd.com is the authoritative source for questions regarding xkcd. In this case, a detailed discussion (including formulas) is taking place on the page for xkcd ...
42
votes
Accepted
How to imagine WiFi signal propagation?
It's more like the walls were semi-transparent glass, if you want to imagine it as light (and even then, you neglect diffraction effects). It would actually be better to imagine it as sound!
But this ...
39
votes
Accepted
How do you make a spherical radio wave?
A result known as Birkhoff's theorem forbids spherical electromagnetic radiation. The statement of the theorem is that any spherically symmetric vacuum solution to Maxwell's equations must be static. ...
36
votes
Accepted
Why do I need to adjust TV antenna on the roof for better video quality?
Picture the radio waves from the TV transmitter as flat horizontal sine waves. You want the antenna to pick up the full width of this wave in the horizontal bars. So you need to point the antenna as ...
30
votes
Accepted
How can wifi penetrate through walls when visible light can't?
Different molecules and different crystalline structures have frequency dependent absorption/reflection/transmission properties. In general, light in the human visible range can travel with little ...
25
votes
Why put a radio telescope in space instead on the Moon?
We've sent three radio telescopes to space so far:
Zond 3 (USSR)
HALCA (Japan, 8 meter dish)
Spektr-R (Russia, 10 meter dish)
Currently, only Spektr-R is operating; the other two have long ceased ...
24
votes
What's the physics behind XKCD #2027 (time between lightning flash and radio wave burst)?
Well, without researching this at all, I’m going to go out on a limb and say it’s due to the difference in refractive indexes between visible light and radio waves in air. Air has dispersion like ...
24
votes
Accepted
What's the physics behind XKCD #2027 (time between lightning flash and radio wave burst)?
Note that the explanation from explainxkcd.com is not entirely correct. Not completely wrong but they make the common error to confuse the group index with the refractive index.
It is the group ...
21
votes
How to imagine WiFi signal propagation?
It's hard to access how 'accurate' an analogy is (i.e. how is this being quantified?). But, I think, there is a simple - better analogy:
WiFi is more like sound in a house. The transmitter is a ...
18
votes
Why put a radio telescope in space instead on the Moon?
@Kyle Kanos is correct, there is a "Radio Window":
allowing ground based operations.
Also: radio telescopes are huge. Arecibo's receiver platform is 900 tons, or 33 full Space Shuttle payloads. The ...
16
votes
Accepted
Why doesn't the ${\rm H}_2$ molecule have a permanent dipole while the neutral ${\rm H}\,{\rm\small I}$ has one?
$H_2$ contains 2 electrons in the same ground-state orbital; by Pauli exclusion, one must be spin-up and the other must be spin-down. The 21cm line is generated in a normal hydrogen atom when an ...
16
votes
Accepted
How does the human body affect radio reception?
Small pocket radios usually have fairly poor antennas in them, because of their small size. But by placing your hand nearby the antenna of that small radio, you are creating a capacitor in which one ...
15
votes
Accepted
Why do we even need RF cavities to accelerate particles?
The key limitation for using a constant electric field (i.e. DC acceleration) is that all materials have an intrinsic breakdown electric field where insulators stop being insulators (true even for air ...
13
votes
Why put a radio telescope in space instead on the Moon?
Some advantages of space over the moon:
The Moon's a lot farther away than near Earth orbit.
If you want to land something on the moon instead of just smashing into it, you need to match the moon's ...
12
votes
Could you use a laser beam as a modulated carrier wave for radio signal?
It's definitely possible to modulate a laser beam to carry radio frequency signals, using any of several different methods. Amplitude modulation, frequency modulation, and phase modulation are all ...
12
votes
Why do we even need RF cavities to accelerate particles?
The reason they'd decelerate is that the space beyond the positive electrode is, in a practical experiment, surrounded by "grounded" stuff (beam pipe, etc.). So, there's effectively a second ...
11
votes
If we have a cosmic microwave background should't we also have a cosmic radio wave background?
Yes, there's a cosmic radio background, just like there's a cosmic optical background, infrared, x-ray, etc. The background from AGN and whatnot that @nielsnielsen mentions doesn't kick in until about ...
10
votes
How can wifi penetrate through walls when visible light can't?
The way light, radio waves or microwaves interact with matter is through electromagnetic interaction with the microscopic charged particles. Different types of excitation can happen with these charges ...
9
votes
Accepted
What is absorption rate of WiFi and Bluetooth RF in water?
The electromagnetic field in a medium gets attenuated exponentially.
$$
\mathbf E = \mathbf E_0e^{-x/\delta}
$$
Where $x$ is the distance the signal has traveled. Since the power of the signal is ...
8
votes
Accepted
From how far away could Earth's telescopes detect Earth-like radio signals?
Using current technology (and by that I mean experiments and telescopes that are available now) we would probably be unable to detect radio signals from Earth even if observed from a distance of a few ...
8
votes
How do you make a spherical radio wave?
"If that is possible, how do you produce a spherical EM radiation?"
A spherically symmetric transverse field is topologically impossible - if it is required to be coherent and linearly polarized ...
8
votes
Why put a radio telescope in space instead on the Moon?
Radio telescopes need to be big - the latest on earth is 1 square km (total area) !
There is no reason to put them in space, (most of) the wavelengths pass through the Earth's atmosphere so there is ...
7
votes
What's the physics behind XKCD #2027 (time between lightning flash and radio wave burst)?
The question of "why" radio waves have a lower speed in air than light might be due to the interactions of radio waves with diatomic molecules ( O1 and N1 ). The radio photons energy are going to be ...
7
votes
Can radio waves be harmful to us?
UHF (Ultra High Frequencies, i.e. microwaves) would be very harmful to us, if our body was subjected to enough UHF energy: it would fry our flesh in the same way it heats up that microwavable 'Chicken ...
7
votes
Accepted
What piece of technology is able to record the exact phase of microwaves?
tl;dr -- baseband it with beats.
The Problem
If you want to accurately record a high frequency time series, you need to sample the data at an even higher frequency, at least twice the target ...
7
votes
How are sidebands generated in an AM signal?
Now you can change the amplitude (AKA the power) of this frequency to encode data, and that's how AM radio works.
Correct. Let's say your carrier signal is $\cos\omega t$, and the message signal you ...
6
votes
Accepted
Photons of a radio wave
We should start with the obligatory warning that a beam of EM radiation is not a swarm of photons. See What is the relation between electromagnetic wave and photon? for more on this.
With that out ...
6
votes
Accepted
Is it okay to sleep with the wifi router next to me?
The wifi router, as well as cell phones and microwave ovens all produce non ionizing radiation. To date, the major concern for non ionizing radiation is its heating (thermal) effect on tissue. It is ...
5
votes
Accepted
Why is UHF so much more popular than other frequencies for radio?
Firstly, a smaller antenna is able to transmit/receive the signals due to the low wavelength of such waves. Secondly, operating using high frequency supports a greater bandwidth as compared to the low ...
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