Questions tagged [thermal-radiation]

The temperature-dependent 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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47
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What are the various physical mechanisms for energy transfer to the photon during blackbody emission?

By conservation of energy, the solid is left in a lower energy state following emission of a photon. Clearly absorption and emission balance at thermal equilibrium, however, thermodynamic equilibrium ...
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Planck's Law in terms of wavelength

I am drawing a blank when it comes to equation transformation. Wikipedia gives two equations for the spectral radiance of black body: First as a function of frequency $\nu$: $$I(\nu, T) = \frac{2 h \...
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What causes a black-body radiation curve to be continuous?

The ideal black-body radiation curve (unlike the quantized emission seen from atomic spectra), is continuous over all frequencies. Many objects approximate ideal blackbodies and have radiation curves ...
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Is fire plasma?

Is Fire a Plasma? If not, what is it then? If yes why, don't we teach kids this basic example? UPDATE: I probably meant a regular commonplace fire of the usual temperature. That should simplify the ...
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Is it possible to focus the radiation from a black body to make something hotter than that black body?

My previous question wasn't specific enough. I'll try to be more specific. Let's imagine we have a hot body let's say 6000K hot that emits lots of thermal radiation. Let's assume 1kW of radiative ...
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Why is black the best emitter?

Why are emitters colored black better emitters than other colors? Why is white a worse emitter?
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Why does thermal radiation only occur at infrared and visible frequencies?

The resources that I've checked out seem to say thermal radiation only occurs in the infrared and visible spectrum. For example my heat transfer textbook and the Wikipedia page on emissivity. In my ...
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How can it be that the sun emits more than a black body?

As far as I know, a black body is an ideal emitter. So how can it be that a non-ideal emitter emits more radiation than a black body? This happens only in a very limited area at around 500nm, but it ...
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The strange thing about the maximum in Planck's law

I read that it makes a difference whether you calculate $\frac{dE(\lambda) }{d \lambda}=0$ or $\frac{dE(\omega)}{d \omega}=0$ in the sense that the maximum energy density with respect to the ...
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How can blackbody radition be explained by quantization?

I don't understand why quantization makes a peak on the blackbody radiation curve (so there is no UV catastrophe) and the relationship between that peak and quantization concept. When the blackbody ...
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What is the real interpretation of Planck's constant and what are its origins?

In the physics texts I have read and from other online information, I gather that Planck's constant is the quantum of action or that it is a constant specifying the ratio of the energy of a particle ...
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What happens to photons after they hit objects?

If I am not wrong when light hits for example white wall most of the photons are absorbed and transformed into heat and few of the photons at certain wavelength are reflected from the object. So white ...
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Possible colors of fire?

I have learnt that depending on the various gases those are involved in the reaction that produces fire, different colors (yellow, red or blue) of flames become visible. I have a question .. what are ...
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If a black body is a perfect absorber, why does it emit anything?

I'm trying to start understanding quantum mechanics, and the first thing I've come across that needs to be understood are black bodies. But I've hit a roadblock at the very first paragraphs. :( ...
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Why are stars white?

That is may be an easy question, but I am not a professional. The sun is a star, and when I look at the sun it is usually yellow. Why are stars in the night white? I suppose it is for the distance. ...
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How does quantization solve the ultraviolet catastrophe?

I understand how classical physics leads to the UV catastrophe. But I cannot understand how quantization solves it. How can quantization prevent the body from radiating a lot of energy? I know this ...
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Why does classical physics imply every mode of vibration should have the same thermal energy?

I've just started reading about photo electric effect here, and my high school level understanding goes something like this : 1) By 1900 we had Maxwell equations and treated light as a wave. 2) But ...
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Why doesn't diamond glow when hot?

In an answer to this SE question, the respondent explains that heating a perfect diamond will not cause it to glow with thermal blackbody radiation. I don't quite follow his explanation. I think it ...
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Why do metals only glow red, yellow and white and not through the full range of the spectrum?

Why don't metals glow from red to yellow to green to blue etc.? Why only red, then yellow and then white? Shouldn't all wavelengths be emitted one by one as the temperature of the metal increases? ...
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The temperature of photon and its energy

Do photons have temperature? If not, does it mean that photon lose energy while travelling through space? As the planets farther away from the sun are comparatively cooler than the one that are ...
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What can we deduce by the fact that mirrors cannot get a ray hotter than Sun's surface?

I think it is quite counter-intuitive that some lenses or mirrors focusing sunlight to a single spot cannot produce a temperature higher than Sun's Surface. What could a scientist deduce from that ...
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Why do moving particles emit thermal radiation?

While answering another question about heat in an atom, the discussion in the comments led to the question of how heat is related to thermal radiation picked up by infrared cameras. The answer is that ...
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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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Why is spectrum obtained by sunlight, said to be continuous?

My teacher spoke about atomic spectra today, and he explained that, unlike the spectrum obtained by analyzing the sunlight, the spectra of atoms are not continuous. I have a question about this - ...
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Temperature on the surface of the sun calculated with the Stefan-Boltzmann-rule

In a German Wikipedia page, the following calculation for the temperature on the surface of the Sun is made: $\sigma=5.67*10^{-8}\frac{W}{m^2K^4}$ (Stefan-Boltzmann constant) $S = 1367\frac{W}{m^2}$ ...
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How exactly does applying the Equipartition Theorem to radiation leads to UV catastrophe?

I'm reading a book by George Gamow, "Thirty years that shook Physics" and have trouble understanding his way of describing the UV catastrophe. In a first part he points out that applying the ...
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Quantum mechanics of thermal radiation

What are the quantum mechanisms behind the emission and absorption of thermal radiation at and below room temperature? If the relevant quantum state transitions are molecular (stretching, flexing and ...
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The Pioneer anomaly finally explained?

Pioneer 10 & 11 are robotic space probes launched by the NASA in the early 1970's. After leaving our solar system, an unusual deceleration of both spacecrafts has been measured to be approximately ...
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How do you change Planck's law from frequency to wavelength? [duplicate]

I have to derive Wien's displacement law by using Planck's law. I've tried but I come to a unsolvable equation (well I can't solve it) anywhere I look online it comes to the same conclusion, you need ...
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Would wearing clothing that is black on the inside and white on the outside keep you cooler?

The Straight Dope ran an explanation of why nomads often wear black clothing - it absorbs heat better from the body. On the other hand, white clothing reflects sunlight better. Is it possible to get ...
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How did Planck derive his formula $E=hf$?

Some time ago I asked my quantum physics lecturer the question: How did Planck derive his formula, the Planck–Einstein relation $$E=hf$$ with constant of proportionality $h$, the Planck ...
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Black Bodies and appearing black

I know a black body is an object that absorbs all wavelengths of light, and also emits all wavelength of light too back out, but if it emits all wavelengths of light, why does it appear black, as all ...
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What is the reason for the shift in balance between neutrons and protons in the early universe?

In the book of The First Three Minutes by Weinberg, on pages 106-107, it is stated that SECOND FRAME. The temperature of the universe is 30,000 million degrees Kelvin [...] The nuclear particle ...
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Kinetic energy as $\pi k_B T$

In the derivation of the Thermal De Broglie Wavelength on Wikipedia, I come across the following: "In the nonrelativistic case the effective kinetic energy of free particles is $E_K=\pi k_B T$ https:...
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How can the black body radiation formula be so general?

In the derivation of the black body radiation formula, there is nothing whatsoever that relates to a particular/specific material. But we nonetheless use this formula for several distinct sources in ...
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Is a black hole a perfect black body?

A black body absorbs all light/radiation in its reach. According to basic laws of physics, the more energy a body absorbs the more it can emit. Therefore, a black body absorbs all energy directed at ...
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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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How is a blackbody spectrum formed in the Sun?

Sunlight can be treated as BB radiation. Why is it a continuous spectrum while the sun contains only a few elements and the radiation from the jumps between atomic levels are discrete? How does the ...
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Heat preserving performance of container relative to content

This question has been addressed in the case of a thermos bottle: Performance of a thermos bottle relative to contents I am asking the question again without the hypothesis that it is a thermos ...
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Black Body Spectrum Plot

I'm Having trouble replicating the Black Body model for sun shown on this plot To my understanding I should only use Planck's formula: $$I(\lambda) = \frac{2\pi hc^2}{\lambda^5}\dfrac{1}{exp\left(\...
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Why do dark objects emit more than lighter ones?

For the purposes of this question, "lighter" and "darker" refer to the absorptive qualities of the objects. Darker objects absorb more light, and therefore appear darker. I'm trying to understand the ...
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All objects radiate energy, but we cannot see all objects in the dark. Why?

We claim that all objects radiate energy by virtue of their temperature and yet we cannot see all objects in the dark. Why not?
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Why does NASA use gold foil on equipment and gold-coated visors?

I've read several websites about equipment covered with gold foil and astronaut helmet visors are coated with gold. However, their explanations are devoid of almost all physics content. Can someone ...
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Explain it to me like I'm a physics grad: Greenhouse Effect

What is the mechanism by which increasing $\rm CO_2$ (or other greenhouse gases) ends up increasing the temperature at (near) the surface of the Earth? Mostly what I'm looking for is a big-picture ...
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Is the Hawking radiation of a charged black hole thermal?

Suppose you have a Schwarzschild black hole of mass $M$ and angular parameter $a = 0$ (no rotation). Question: is it possible to throw a charge $Q$ at a faster rate than it will be re-radiated? Will ...
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Is a suit that hides a soldier's heat signature fundamentally possible?

I recently played "Crysis", a game where the protagonist wears a suit that allows the player to hide both himself and his heat signature. Then I watched Iron Man 3, where a kid suggests that Tony ...
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Entropy of radiation emitted into space

In several papers I see something equivalent to the following expression for the entropy of radiation given by an astronomical object such as the Sun (assuming the object can be approximated as a ...
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Why would different metals glow red at different temperatures?

According to everything I've been taught about incandescence and black-body radiation, and some quick Googling to confirm I'm not crazy, just about everything, regardless of composition, should start ...
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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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I don't understand why a cavity with a hole is a black body [duplicate]

It makes no sense to me that a cavity with a hole is identical to a black body. Sure, the cavity will probably be a near perfect absorber, but I don’t see how it will be a perfect emitter as well. ...