# Tagged Questions

The temperature-dependant emission of electromagnetic waves. Combine this tag with [tag:thermodynamics] for a macroscopic view or [tag:quantum-mechanics] for a microscopic explanation.

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### Black body emission by a dielectric slab [on hold]

I need to calculate the radiation emitted by a rectangular dielectric slab (glass) at temperature $T_0$. The slab has a thickness $t$, sides $a,b$ and a dielectric constant $n(\lambda)$. How can I do ...
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The emissivity of a hemi-sphere varies as: $$e=E\cos\theta$$ What is the ratio of emissive energy between the given sphere and a normal black body sphere.
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### Conservation of energy when focusing black body radiation on another black body

This question about whether or not it is possible to focus black-body radiation to make something hotter than the radiation's source was answered mostly negative: the second law of thermodynamics and/...
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### How can IR cameras see things colder than the camera?

I’ve read that only cold-blooded animals develop IR vision to spot warm prey because a mammal would blind himself with his own heat and not be able to see anything. But I’ve seen a FLIR camera, ...
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### Is the universe inside some huge black body? [closed]

I've been reading about the black body radiation. A perfect black body can absorb all possible wavelengths- which can be put from zero to infinity. But that will depend on the size of the black body ...
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### Could a human feel the black body radiation of another human standing behind her?

I've been thinking about infrared radiation and noticing more and more how the human skin seems actually pretty sensitive to it. You can easily feel a bonfire from several meters away, far away from ...
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This is my understanding of the black body radiation: a black body is heated. The energy from the heat causes the electrons in the body to oscillate, so they will emit electromagnetic waves. If we ...
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### Work done in adiabatic reversible process

I was solving a problem on turbine.the steam works on turbine adiabatic reversibly .Is change in enthalpy or is it change in internal energy which equal this work?
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### Total spin of many photon thermal state [closed]

I need to find the total spin of a large spin 1/2 ensemble in thermal state: I'm trying to find a formula for $\operatorname{Tr}(S^2 \rho)$ where $\rho=\frac{1}{Z}e^{\beta S_z}$ is the thermal state ...
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### Black body radiation and number of photons emitted

Usually the black body radiation (at a certain temperature $T$) is given by $$\rho ( \nu ) = \frac{8 \pi h \nu^3}{c^3 \left( e^{h \nu / (k_B T)} - 1 \right)}$$ This quantity $\rho ( \nu )$ should be ...
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### How / why do infrared thermometers work?

How can reading the intensity of infrared radiation coming from an object let you calculate its temperature? Could any wavelength be used to measure temperature - why infrared, and not, for instance,...
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### Is the visible light spectrum from “red-hot glass” at least close to Blackbody Radiation?

Briefly, for visibly transparent materials like glass, you can see through then even while they are glowing red hot. Most glasses have plenty of absorption in the IR, so there is plenty of absorption ...
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### Thermodynamically possible to hide a Dyson sphere?

You build a Dyson sphere around a star to capture all its energy. The outer surface of the Dyson sphere still radiates heat at much higher temperature than the cold space background, so you're easy to ...
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### Problem on Emissivity and absorptivity

I have read the radiation chapter, where I have been introduced with the terms emissivity and absorptivity. emissivity tells about the ability to emit heat energy as thermal radiation compare to a ...
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### Inequality of dimensions in Kirchhoff's law

I have read about kirchoff's law of radiation ,which tells that ratio of emissive power of a body to the absorptive power of a body is constant and it is equal to emissive power of black body So Qe/...
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### Can an electromagnetic wave be split into multiple longer wavelength waves?

Is there any way to convert short wavelength (visible light/near infrared) electromagnetic waves into multiple long wavelength waves (microwave) of near equal total energy? Heating a blackbody with ...
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### Why isn't cosmic microwave background radiation on Earth drowned out by thermal noise

The Penzias/Wilson experiment, which detected the CMB, doesn't sound feasible given thermal noise. When Penzias and Wilson reduced their data they found a low, steady, mysterious noise that ...
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### Heat loss by radiation reference

In our lab practice, we measured the rate of heat loss of a copper plate in atmospheric pressure and the rate of heat loss of the same plate in high vacuum ($\sim 10^5 torr$). So all I could find for ...
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### Calibrating the relative intensity of a spectrometer with a blackbody source?

I am trying to compare the relative brightness of $H \beta$ and $H\alpha$ lines using a CCD spectrometer. In order to correct for the different grating efficiency at the two different wavelengths. I ...
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### Does fusion continue to happen as a star collapses into a black hole (beyond our view)?

I know that we can't actually see a black hole form, because of time dilation, and we would see a frozen image just before the final collapse past the event horizon, slowly redshifting to nothing... ...
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I have some questions about blackbody radiation. In the classical calculation, we assume every mode has the same energy, then every mode should have the same probability, then why the curve of ...
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### Calculating new temperature of an object when air temperature changes

I'm trying to calculate the new temperature of an object when the air temperature around it changes, given a period of time. Basically I get periodic readings from an air temperature sensor in a ...
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### light effects of electric current

In german high school textbooks and curriculua, the distinction of following main sorts of effects of electric current is usual: magnetic, heat, light and chemical. My question is, whether the glow ...
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### An ideal black - body at room temperature is thrown into a furnace. It is observed that? [closed]

An ideal black - body at room temperature is thrown into a furnace. It is observed that? (A) initially it is the darkest body and at later times the brightest. (B) it is the darkest body at all ...
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### Moving heat problem: instant toast?

Toast-making usually takes a couple minutes using something like a conventional toaster oven to brown the surface of a piece of bread, making toast. However, this duration may be inconvenient. What ...
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### Trouble in deriving the Rayleigh-Jeans formula from the steps shown on HyperPhysics site

Hi I am very new to this aspect of physics and I am having trouble with the derivation of the Rayleigh-Jeans from the steps shown at the hyperphysics web site. I have emailed Dr. Rod Nave who is ...
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### How does radiative cooling between the earth and space work?

On a chilly (but above freezing) temperatures on a clear night, you can freeze water outside because of radiative cooling. By what mechanism do warm bodies on earth actually lose heat to space? ...
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### Why do High Pressure Gases produce a Continuous Spectrum?

I am aware that low pressure/density gases produce an emission spectrum as there are specific energy transitions that the electrons can make, emitting certain frequencies of EM waves. However high ...
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### Planck's distribution and Bose-Einstein distribution?

If the application of the Bose-Einstein distribution is in blackbody radiation, then what is Planck's distribution? Are they same? How did Planck know that he should use a Bose-Einstein distribution ...
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### Finding the exponent of $\lambda$ in Wien's displacement law

I am reading this paper on a short history of the $T^4$ radiation law. In particular, on page 5, By assuming that the wavelength of radiation emitted by a molecule was a function only of its ...
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### understanding thermal radiation in a conductor, gas and insulator

Context: I was in my bliss of ignorance and happiness when I was taught that quantum mechanics was about nice discrete values of energies. Now I am introduced the idea of Fermi Energy in a block of ...
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### Cooling down to absolute zero by radiation

Consider a system consisting of a gas, it is put in a container which is permits transmission of all kinds of electromagnetic waves. If this system is isolated and put in a perfect vacuum, and left ...
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### Can we determine the surface temperature of stars other than the sun by using the black body radiation theory?

It is well known that the surface temperature of the sun can be determined by fitting the solar spectrum to the black body radiation spectrum. Is this scheme feasible for other stars? Possibly the ...
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### Reaching equilibrium in a blackbody and light-matter interaction

Suppose we have a metallic cavity maintained at a fixed temperature. Suppose we start with any distribution of radiation that is not in equilibrium with the container. Gradually, when the equilibrium ...
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### How do Black Bodies Absorb and Emit Radiation? [duplicate]

I have learnt how the gases of elements are able to absorb only certain wavelengths of EM radiation corresponding to the energy transitions between energy levels of orbitals. Furthermore, these ...
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### Radiative Cooling of metal object

We have this experiment where a metal bar is heated and then we have to make a model for the cooling that occurs. We get numbers for how long it takes the metal bar to cool from 200 to 100 degrees ...
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### What temperature can you attain with a solar furnace?

A solar furnace is a device that concentrates the sun's light on a small point to heat it up to high temperature. One can imagine that in the limit of being completely surrounded by mirrors, your ...
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### Historic derivation of Wien's law

Every book I've read, including a lot of websites, Wikipedia, etc, say that Wien derived this: $$\rho_\nu(T)=\rho(\nu,T)=\nu^3f\left(\frac{\nu}{T}\right)$$ Being $\rho_v(T)$ the spectral enegy ...
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### Radiation-Glossy Black vs Matte White

Well, studying the chapter Heat I know that a matte black body radiates heat more than a glossy white one but can anyone give me an answer about what happens when its a glossy black body and a dull ...
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### Maximum temperature possible on earth

What is maximum temperature that can we have on earth on a single day? Lets say an air mass is static over an area and there is no way for air mass to move, sun warming it up would increase ...
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### Diminishing solar temperature and its effects on earth

This is a hypothetical question; considering both the earth and the sun as black bodies. If the temperature of the sun decreased N times, what would be the effect on the radiation intensity received ...
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### Is Moon too hot?

I find it puzzling that Moon's maximum "daily" equatorial temperature is almost 400K. Earth's theoretical black body temperature would be 279K at 1AU, and Moon is the same distance from Sun, yet its ...
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### Do bifacial solar panels operate at lower temperature than normal solar panels?

Bifacial solar panels have a glass backplane instead of the usual aluminum and are designed to absorb and convert to electricity light incident on either face. Many manufacturers claim that their ...
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### Thermal radiation of a nitrogen sphere

Let's imagine a : sphere of 100% pure nitrogen (N2), (edit: 1 m diameter) with a constant volume (edit: using a kind of "magic forcefield") (edit : at 1 bar) in the void far from any light source (...
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### If an object has a temperature, does it have to radiate?

I'm reading through a powerpoint presentation about Hawking Radiation (HR). They are explaining all of the reasons that built up to the postulate of HR, and one of the reasons is that if there is a ...
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### Radiative equilibrium of a relativistic black body

A black body in empty space in the CMB rest frame should (discounting starlight) come to an equilibrium temperature of around 2.7 K. However, if the body is moving relative to the CMB frame, one side ...
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### Is it possible to produce higher temperature than the light source with passive device only?

Suppose the light source is a black body with average temperature 2000K, and I want to use it to heat another black body to 2500K, via passive devices only. Here passive device means lens, mirror, ...
Suppose you were inside a thick spherical shell of inner radius $R$, which was a perfect black body at some temperature T. What would be the power a sphere of radius $r$ would absorb located inside ...