# Tag Info

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What is the temperature of the clear night sky from the surface of Earth? It's much closer to 273 K than 2.73 K. The answer depends on the surface temperature, the humidity, the temperature gradient through the atmosphere, and what exactly you mean by "the temperature of the clear night sky". The Swinbank formula provides an ad hoc expression for the ...

4

The H.E.S.S. Gamma ray observatory in the Namibian desert has sufficient collecting area that it is able to detect (via Cerenkov radiation) very rare high energy gamma rays that were simply not available to space observatories with comparatively low collecting areas. H.E.S.S. detects high energy gamma rays from all sorts of astrophysical objects and ...

1

All you need to do is conserve energy and momentum in the lab frame. Firstly you conserve energy in lab frame: $$E_{\gamma 1} + E_{\gamma2} = E_{\pi} = 1.3GeV$$ Then you work out what the pion's momentum was (still in the lab frame) using the mass-energy-momentum relation where the $E_\pi$ is the total kinetic and mass energy: ...

6

Intensity has units of watts per area: $$\left[I\right]=\rm\frac{W}{ m^2}$$ where the area is the surface area of the emitting source (in this case, the sun). This tells you the total amount of radiation present (over all wavelengths). The extra factor of 1/nm in your plot gives the spectral irradiance: $$\left[\mathcal E\right]=\rm \frac{W}{m^2\,nm}$$ ...

2

$W/m^2$ would be the total energy emitted, regardless of wavelength. When you use $W/m^2/nm$ you are explicitly saying that it corresponds to a specific part of the spectrum (nm is a unit of wavelength). Which is what the graph you posted is showing. The first one is called "irradiance", the one plotted here is called "spectral irradiance". For more details ...

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theoretically yes. If the white garment on the outside is opaque enough, it will block more of the sun's light from reaching the inner black garment. The black garment will actually help you keep cooler if you have both garments made properly. This is because black does not actually absorb heat, it traps it therefore not allowing the "cooler" air to heat it ...

2

This article covers the question Radiation is divided into two categories - ionizing radiation and non-ionizing radiation. Ionizing radiation is radiation with sufficient energy to remove electrons from the orbits of atoms resulting in charged particles, and it is this type of radiation that is evaluated for purposes of radiation protection. ...

2

It is all about wavelength versus tunnel diameter. The wavelength of GPS is about 20cm it would happily propagate in any normal tunnel if it could get in but the earth and other structures absorb it. AM radio (600kHz - 1500kHz) cannot propagate in any normal tunnel because the wavelength is too long (500m-200m) relative to the diameter, and thus gets ...

0

Radio waves are just light of a different frequency. Yes, they are slightly better at reflecting off various surfaces than light, but they still generally travel in straight lines. They don't flow or fill space like a fluid, so there is no reason to believe they'd be good at following you into a tunnel. You generally need line of sight (or something close to ...

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Both of these technologies rely on radio frequency waves, which are blocked by dense matter eg, a hill, a building.

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Both single beta-decay and double beta-decay may occur with $e^+$ as well as $e^-$. However, in both cases, the emission of $e^-$ is predicted to appear (and in the single beta case, is also observer to appear) in a larger number of processes essentially because it's energetically easier for neutrons to decay to protons plus electrons; than it is for ...

1

Confusion about Length Contraction [...] if one object is moving at a speed relative to another object, shouldn't this movement affect [...] Talk about "length contraction" (or "time dilation") is inherently confusing; it is improper and should be avoided. In the typical "cosmic ray generated atmospheric muon" example we have the following unambiguous ...

1

Length is contracted the other way, we just don't notice it because muons are pointlike particles (as far as anyone knows), and a length of zero is still zero no matter how much you contract it. If you had a beam of muons spaced a fixed distance apart (and yet somehow their "decay counters" only start at $50\text{ km}$ altitude), then it would be a ...

2

Don't confuse time dilation and length contraction,(even if length contraction is a consequence of time dilation) because there is no length contraction for the distance between two reference frames. The length (or distance) must be found in one reference frame, and the observer must be in another reference frame, he may not be part of the reference frame of ...

1

Short answer: Yes, length contraction is a symmetric transformation between reference frames. However... You may not like this answer, but I think it's far easier to regard muon decay via the concept of time dilation rather than length of contraction. Reason is, from the standpoint of the muon it's not moving (and yes the Eath is moving but it's not ...

1

Well, Lorentz transformation and whole special relativity gives us short qualitative answer - time dilation and length contraction are very similar. Sitting on Earth we see muons with slower decay rate, because of time dilatation. Being a muon, we calculate time in our own frame of reference - so to keep calculations clear, we need to contract length ...

0

Once the gammas are produced, they do not carry particular information. There is just a certain probability, measured by a differential cross section, that, if they scatter again, they produce $e^+,\,e^-$ as in the initial pair.

1

First off, Strontium accumulates in the bones replacing Calcium, so you can't easily get rid of it. Second, the most radioactive natural food we eat is the banana, at about 130 becquerels per Kg. So that 1L of "hot" water really is quite nasty if you drink it and a significant portion of the Sr90 ends up in your bones.

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There are many types of nuclear decay, and many techniques for estimating half-lives. For beta decay of states in spherical nuclei, calculation of decay rates is a classic application of the (spherical) nuclear shell model. For gamma decay, there are generic estimates that are based on the energy and multipolarity of the transition. (The term to google on ...

3

Here is a table of isotopes versus lifetimes the color code of the lifetimes on the right hand column: Isotope half-lives. Note that the darker more stable isotope region departs from the line of protons (Z) = neutrons (N), as the element number Z becomes larger Modeling a nucleus is a many body problem and also a many forces problem. There exists ...

3

The transition probability per unit time of a nucleus from an initial state i to a final state f, representing the decayed system, is modeled by Fermi's Golden Rule: $$\lambda=T_{i\rightarrow f} = \frac{2\pi}{\hbar}\left|\left\langle i\left|H'\right|f\right\rangle\right|^2\rho$$ Where $T_{i\rightarrow f}$ is the transition probability from state $i$ to state ...

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The constant is a function of the stability of the nucleus, and is experimentally determined for every isotope. In other words - every kind of nucleus has its own value of $\lambda$ and there is no way (that I know) to get an accurate value for it, other than measurement. But there are some nuclear physicists roaming who will put me out of my misery, I'm ...

1

Is it possible to heat up, let's say, a blob of mud, golf ball size, covered in AL foil, with IR? Possible? Sure. Also, how efficient would that be? Metals are weakly coupled to the electromagnetic field, i.e. they reflect most of what hits them, at and above visible wavelengths. That means that most of the incoming radiation just bounces off, so ...

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