# Tag Info

49

Electromagnetic radiation will continue to travel until it is absorbed. Some of your wifi signal is escaping to space where it may continue traveling for a very long time. However, the strength of your wifi signal will degrade with distance according to the inverse square law. So if you double the distance between your device and the wifi transmitter, your ...

13

Signal-to-noise ratio In addition to what others have said, it is very important to understand the difference between just detecting something and decoding a useful signal from it. The CMB is essentially random noise – in fact, that's how it was discovered in the first place! Still, in normal conditions it is easily drowned by other noise sources and was ...

12

Yes, the atomic hydrogen is probably mostly left over from the Big Bang. [Edited to add: Not sure how much that is true and how much present-day atomic hydrogen is the result of recombination.] And, yes, ${\rm H}_{2}$ does get dissociated by high-energy photons -- and also by cosmic rays, which can penetrate dense, dusty clouds that block most of the high-...

10

There's higher quantity of atoms in your 20cm wall than there is in the 13.8 billion light-years travelling to the CMB, so the wifi waves hit atoms on their travel. Space has an average density of 5.9 protons per cubic meter, that's 10^-25 g/m3, and there are only 1.22*10^26m to travel to the CMB source. The CMB is an omni directional transmitter source, it'...

9

No. But the reasons are biological, not physical. Your eyes work by the interaction of electromagnetic radiation with certain molecules ( rhodopsin which consists of the protein opsin linked to 11-cis-retinal, a prosthetic group.) These molecules are tuned to detect light of particular wavelengths. But they couldn't be tuned to detect radio-waves, since ...

8

Your question may ulitmately be about the physiology of the eye, which is off-topic here. The spectrum of the Sun seen low on the horizon is quite different to the spectrum of an M-type red dwarf. The reason that a red dwarf is red, is not just that it is cool, but that there are great chunks of the spectrum that are absorbed by molecules in the photosphere ...

6

The Physics SE answer (or the part quoted) was incorrect. The photon does not have to have "precisely" the right energy to cause a transition. The reality is that there is a non-zero probability of causing a transition at all photon energies, but the probability distribution is sharply peaked at the energy we calculate to be the energy difference ...

5

You cannot have free protons without electrons. Plasmas, in general, are electrically neutral. It is usually electrons that dominate the scattering (note that a point-like charge cannot absorb a photon and conserve energy and momentum) in a plasma at low photon energies. That is basically due to their much lower masses (classically you can think of the ...

4

There are two effects causing this: The relevant quantity for determining whether or not a photon interacts with a particle is the ratio $$x \equiv \frac{2\pi r}{\lambda},$$ where $r$ is a size of the scattering agent. Rayleigh scattering When $x\ll 1$, we're in the Rayleigh regime where the wavelength is so long compared to the particle that the ...

4

Surely the sun possesses calcium in its atmosphere, as well as in its bulk volume. This plot, based on the data published in Asplund et al.,(2009), shows what elements can be found in the solar atmosphere: And we can read off that the abundance [Ca]/[Si] = 0.1 for example. Elements in stellar atmospheres can occur both in absorption and emission in stellar ...

4

This is one of those questions that is easy to state but complicated to answer - and this won’t at all be a complete answer, but mostly a quick outline of some important factors to consider and terms you might search for in order to learn more. The question of why the interstellar medium (ISM) has the structure it does is a long-standing one, and one that a ...

3

'Absorption' lines are caused by resonance scattering (scattering the radiation out of the line of sight, see illustration below), and resonance scattering has a very large cross section of roughly $10^{-12} cm^2$. This means that even for a thin layer of 10km ($10^6 cm$) you need only a density of >$10^6 /cm^3$ of an element for the layer to become ...

3

The strength of an absorption feature in the stellar spectrum is dependent on the amount of that element that is in the photosphere but it also depends on the atomic structure of the element and the conditions of temperature and density in the photosphere. For example the CaII lines need there to be singly ionised calcium ions in the photosphere. This ...

3

If we take 1 atmosphere of optical depth to mean looking though the Earth's atmosphere at zenith, then the optical depth to scattering is small - probably of order 0.3 for blue light and much smaller (according to $\lambda^{-4}$) for red light. That means that when the Sun is at zenith, most of the light reaches the ground but some blue light is scattered ...

3

The picture is a mocked-up fake and is not an actual picture of the solar spectrum. You can easily see this because the black "Fraunhofer lines" extend beyond the spectrum and H alpha should have an appreciable width. The table is massively incomplete. It list only a tiny fraction (the strongest) absorption lines in the solar spectrum. There are ...

1

Yes, and the reasons are both physical and biological. Our eyes use molecules that can be excited by electromagnetic visible light waves (wavelength 0.4 to 0.7 microns roughly) and those excitations can then be converted to other molecular signals and eventually depolarization of nerve cell membranes ("neurons firing"). Snake eye-pits use ...

1

I'm uncertain of the answer; there seems to be some uncertainty involved in the mechanism, as if there were some kind of principle involved ;-) I'll never get quantum mechanics, but that's the nature of QM; it simply doesn't work the same way we think the "real world" works. I think the challenge question to this question is "Can a photon even ...

1

While @planetmaker's comment is true if the lines come from the same source, you can have lines emerging from different physical processes which still appear to come from the same location. An example is absorption (or more rarely emission) lines from galactic winds, which are typically blueshifted with respect to the "systemic" redshift, i.e. the &...

1

Reddening (or the fact that blue light is more extincted than red, causing objects to appear more red) is due to the interaction between the light and the dust grains and gas molecules it is going through, and is caused by the relative size of the dust grains and of the wavelength. Indeed, dust grains are very effective at scattering light which has a ...

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