# Tag Info

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The IR radiation from a gas heater is dangerous, but not for the reasons you seem to think. Each year nearly 500 people die as a result of space heater radiation http://www.nfpa.org/press-room/news-releases/2010/space-heaters-involved-in-79-percent-of-fatal-home-heating-fires. Granted, in the US, space heaters are almost always electric, but I'm sure there ...

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Solar radiation is pretty close to $1kW/m^2$. That's already borderline problematic for a human who is completely exposed. I would guestimate with a giant foam hand that $2kW/m^2$ for more than a few minutes is more than enough to give humans a bad time. You could, of course, test this experimentally with a 100W light bulb. What's the closest distance that ...

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This is a standard particle-vortex duality transformation. The idea is there exists a "dual" description of a superfluid, in which the bosons (to be precise, the Goldstone mode) is described by a 2-form(tensor) gauge field, and the vortices are now string-like "matter" charged under the gauge field. The effective action for phase fluctuations (i.e. Goldstone ...

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Ionizing radiation loses energy in matter by creating electron-ion pairs. Suppose you have an 1 MeV charged particle stopping in a silicon crystal. The first ionization energy for free silicon atoms is about 8 eV. The ionization energy will be a little different for silicon atoms on the lattice, but not grossly so: your 1 MeV charged particle is going to ...

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According to the Review of Particle Physics (Section 33.7.4 of the 2014 edition) there are two main causes of radiation damage for electronic devices: Bulk damage due to displacement of atoms from their lattice sites. This leads to increased leakage current, carrier trapping, and build-up of space charge that changes the required operating voltage. ...

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The big mystery is: why should nature prefer one direction over another? And the answer is still unknown. From wikipedia: The experiment's purpose was to establish whether or not conservation of parity (P-conservation), which was previously established in the electromagnetic and strong interactions, also applied to weak interactions. If ...

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For those wondering why zinc sulfide is important, I will note that "a zinc-sulphide screen in vacuum" is specifically called out in the original Geiger and Marsden papers on alpha particle scattering. It was already well accepted as the coating for the early cathode ray tubes, and zinc sulfide would become one of the main phosphors for CRTs for television. ...

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Nuclei have a series of discrete energy levels (somewhat analogous to electronic energy levels, but the details are, not surprisingly, different). Examples of these so-called Energy Level Diagrams can be found at, e.g., Triangle Universities Nuclear Laboratory. So, a simple alpha decay will go from one level in the parent nucleus to one level in the daughter ...

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I'm sure this isn't what you had in mind, but... Reflected optical light (I can't think of a reason that UV wouldn't be just as good, provided there is enough UV radiation around to be reflected, e.g. sunlight) is 'radiation from a human'. A picture is a pretty good way of identifying humans. Provided the picture is sufficiently detailed (for instance, ...

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To make an object "invisible", one important thing you have to do is to stop it from blocking the light behind it from the observer's perspective. If you have an opaque object that emits/reflects no visible light, then what you will see is a black silhouette of the object. The only way it will be invisible then is if it's against a completely black ...

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CMB hasn't a frequency but a typical black body frequencies x radiances distribution. With the Planck law, the curve distribution gives the temperature of the radiation. I found this image in a previous question Relationship between temperature and wavelength? As you can see, each temperature has a typical curve. Yes, red/blueshift affect the ...

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No. Light travels at the speed of ... light, when measured locally in inertial reference frames. And the relationship between wavelength and frequency is $\lambda = c/f$. As the universe expands, the wavelength of the cosmic microwave background photons is "stretched" and thus their frequency must decrease by the same factor of $(1 + z)$, where $z$ is the ...

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Has the frequency of CMBR changed at all since the beginning of the universe? The usual answer is yes. It's thought to have redshifted by a factor of a thousand. But there is an issue: conservation of energy. Where did the energy go? This is an intriguing thread to pull, because we don't know of anything that's in breach of conservation of energy. There ...

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Theoretically, the CMBR is what we see of the early visible universe. If we could be there to observe it, we might see much higher frequency radiation. However, due to the expansion of the universe, the wavelengths got stretched out and the frequency redshifted. Or perhaps you can say in our reference frame, we happen to measure these photons to be ...

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To summarize: hotter => bluer, but more radiant => brighter. Something which is very hot, but invisible, would need to be small and have an energy output proportionate to the lower surface area. An energy source of a given number of watts is indeed easier to "hide" if it emits mainly at higher frequencies. This is what makes cobalt-60 dangerous when ...

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1) It's true that the peak wavelength for a black body decreases with temperature. But let's say you want to know what temperature has what peak wavelength. Well, you can Google on "peak wavelength temperature calculator" and try for yourself. But I'll give you the short form. Since visible light is in the range of 400 to 700 nm, your body would have to be ...

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Planck's Law gives us the intensity of black body radiation as a function of temperature: $$B(\lambda,T)=\frac{2hc^2}{\lambda^5}\cdot \frac{1}{e^{\frac{h c}{\lambda k_B T}}-1}$$ If we plot a normalized plot of this curve for different temperatures, you see the following: As you can see, it does look like the higher temperatures make the relative ...

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You're right that as the temperature increases, shorter wavelengths receive a higher proportion of thermally radiated power, and longer wavelengths a smaller proportion, because of the shifting Boltzmann distribution of your molecules' kinetic energy, and therefore the shifting power spectrum of the light they emit. However, most of the objects you see ...

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