Questions tagged [thermal-radiation]

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

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What exactly is Planck's constant? how did they calculate it?

What exactly is Planck's constant? I'm a pharmacy student and I've googled this question but couldn't find an answer I'd understand. It would be so great if somebody could tell me how we calculated ...
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What is thermal radiation on a molecular level? [duplicate]

I’ve recently been scouring my Finnish high school level textbooks and online physics forums for an answer to the following question: ”How is thermal radiation created on a molecular level?” or ”What ...
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Where do Photons in Hot “Box” Come From?

I've read through some related questions, like here, but these don't quite answer my question. Say I have an oven at room temperature, and I raise the temperature to 400 K. During this process, the ...
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If the moon became a black hole, how much lower would the temperature be on Earth's surface?

I read that if the Sun became a black hole the Earth would become colder, but the effect would be negligible because the mass of the hole is still there so there would still be tides. But, how much ...
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Planck`s law for an interval of wavelengths

Planck's law for spectral radiance of a black body for a wavelength: $$P=\frac{2hc^2}{\lambda^5}\frac{1}{e^{hc/\lambda k_bT}-1}$$ If I want to know the spectral radiance for an interval of wavelengths,...
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If all objects emit IR radiation, would it be sufficient to place an IR mirror behind an object to propel it forward?

I'm assuming objects don't move all the time because they emit IR radiation in all directions. If we have an IR mirror behind the object to focus their direction, would this propel the object forward?
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Generalised thermal equilibrium condition

The Newtonian condition for thermal equilibrium for a static system, $T = constant$ being $T$ the temperature, should be generalised, for a curved space, by the condition: $T(-g_{00})^{\frac{1}{2}}=...
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Big problem with the standard derivation of Rayleigh-Jeans law + reason for this standard approach + solution

The problem: The standard derivation of the Rayleigh-Jeans law, in a cubic reflective cavity with a small blackbody in it, would have you believe that the cavity has an energy density of $kT$ for ...
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The temperature of the surface of the sun

If I calculate the temperature of the Sun by replacing the effective frequency of the Sun (598  THz)  in Planck's curve for Blackbody radiation, the result will be 10170 K. Still, in Wien's ...
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Nyquist noise and thermal equilibration

Let there be given two identical lumped element resistors $R_1=R_2$ whose heat capacities are also equal and given $C_1=C_2$. We assume the resistors are attached to thermostats, one at temperature $...
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Two identical solid cylinders, which transfers energy faster?

Two scenarios: Solid metal cylinder of $43^\circ$ C is placed in room of $23^\circ$ C Identical solid metal cylinder of $3^\circ$ C is placed in another (identical) room of $23^\circ$ C at the ...
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Blackbody radiation from principle of least action

I was recently studying Blackbody Radiation and the principle behind it (as far as used in the Plancks original paper) is to find the energy distribution which maximizes the number of ways in which ...
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Discrete Cavity Radiation vs Continuous Blackbody radiation, violation of the 2nd law of thermodynamics?

Blackbodies emit a continuous spectrum of radiation, whereas a cavity with reflective walls at thermal equilibrium contains a discrete spectrum. According to Kirchoff, "smoothing out" the ...
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Violation of the second law of thermodynamics with thermal radiation

If we have two cubic cavities both with perfectly reflecting walls, one much large than the other, both with a blackbody in them, then when they are both in thermal equilibrium with their surroundings ...
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Blackbody radiation vs spectroscopy

If a body (eg a metal) is glowing red hot then its temperature can be approximated solely based on the colour. But what if that colour is not due to blackbody radiation but due to excitation of ...
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Is blackbody radiation due to electrons transitioning from higher levels to a lower level? [duplicate]

Yes please answer my question as I am not sure if I am right. Thanks in advance.
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Why greenhouse's inside temperature never been infinitely hot?

From what i understand recently, greenhouse gases traps the sun heat (infrared radiation) on earth, by making it harder to dissipate back into space, reflecting most of them again. The exact same ...
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Physics of an infrared thermometer

The thing about infrared thermometers that bugs me is how can you get the same temperature reading regardless of the distance to the object. Shouldn't there be a difference when measuring from two ...
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What is the origin of the Sun light?

The Sunlight is an electromagnetic radiation. Is it known what is the origin of this radiation? Can it be adequately described by classical electrodynamics (Maxwell's equations) as a motion of ...
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Kirchhoffs' radiation law

I have a doubt regarding Kirchhoff's law of thermal radiation. From what I found online Kirchhoff's law says that the monochromatic absorptance equals emittance in thermodynamic equilibrium, in a ...
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Planck's catastrophe?

In deriving Planck's blackbody formula, the number density of normal modes (per unit frequency$^\dagger$) is found, given by $$ N(\omega)=\frac{V}{\pi^2c^3}\omega^2, $$ where $V$ is the volume of the ...
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Cavity Radiation, why should only standing waves persist in the cavity?

"Because when the box is in thermal equilibrium, there can never be any electric field at the walls for it would shake the charge in the wall around, changing the temperature, contradicting the ...
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Does the formula for total energy emitted by black body considers only 1 solid angle, or the whole sphere?

According to Stephen-Boltzmann law Wikipedia article, the total energy radiated per unit surface area of a black body across all wavelengths per unit time is: $$j=\sigma T^4$$ So the total energy ...
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Can a black hole be considered as a gray body?

The emissivity of a gray body is constant but is considered to be less than 1. Since the black hole is considered near perfect black body but not a perfect black body its emissivity will be less than ...
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What exactly was the “classical model” of black-body radiation, and what assumption about it made it wrong?

While studying black-body radiation, I saw that most of the textbooks and videos I watched mentioned that there was a contradiction between the classical model of black body radiation and ...
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Can hypervelocity impacts be an efficient EUV source?

Imagine that two small pieces of rock hit each other at a very high velocity (for example, small meteoroids moving at orbital velocity in opposite directions). How much of the impact energy could be ...
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Why the extraterrestrial solar spectrum is higher than blackbody spectrum? [duplicate]

I am reading "Radiation Heat Transfer by Michael F. Modest(2003)" and the author talks about how the solar spectrum can be approximated by black body radiation at 5777K. I'm finding the plot ...
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If the Sun's temperature is around 5000K to 6000K which means it is a yellow star, why does it appear white in space? [duplicate]

If the thermal radiation of stars is close to the thermal radiation of a black body the sun should appear yellow but yet it appears white because of emitting all of visible wavelengths which combine ...
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What variables affect an object's absorption of invisible light?

Say I have two objects (we'll call them object $1$ and object $2$) of the same material but of a different colour, placed at the same distance away from another object (we'll call object $3$) that ...
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How does information loss creep into the Bogoliubov-coefficient calculation of Hawking radiation?

I understand the basic idea behind the 1975 Hawking's paper; in the last section there are some comments on the final product of total evaporation. But there is no mention of information loss. I know ...
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Can we apply Stefan's law to find how much energy a body absorbs?

Stefan's law tells gives an expression for thermal radiation emitted per unit time by a body of surface area A and temperature $T$ $$ u = \sigma A e T^4$$ In my book, it is written that in thermal ...
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From Rayleigh-Jeans law to Planck's distribution

Our study project does not include it, but I would be curious to see how the Planck's distribution for the black body emission was obtained out from the Rayleigh-Jeans law. Could you tell me some web ...
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Why do blackbodies emit all frequencies of light? [duplicate]

In Quantum Mechanics - Concepts and Applications by Zettili, he states that an ideal blackbody can emit light across the entire spectrum. Why is this? In other words, what is the physical process that ...
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If I'm sitting in my bedroom and everything is in thermal equilibrium, is the spectral content of my room approximately a blackbody spectrum?

Maybe a bit of a stupid question, but I'm imagining that my room is effectively a cavity with no leakage of light and minimal reflection. If I get a spectrometer and hold it out will it have the ...
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Will “water” glow at very high temperatures?

I was chatting with my kids about how incandescent lightbulbs work, clarifying that they don't so much "burn very slowly" (their original understanding) so much as they emit photons "...
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What was the question to which Max Planck gave an explanation and how is it related to blackbody radiation?

Quoting from my text book - "several attempts were made to predict intensity as a function of temperature/wavelength". This line was preceded by the explanation for blackbody radiations and ...
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What does it mean when we say “a blackbody is in thermal equilibrium with it's surroundings”?

Quoting from my text book - "a blackbody is a perfect radiator of radiant energy. It is in thermal equilibrium with it's surroundings and radiated as much energy per unit area as it absorbs from ...
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Why highly polished silver ${\rm Ag}$(111) doesn't glow upon heating?

Why highly polished ${\rm Ag}$(111) crystal surface, doesn't glow upon annealing upto 600-700C, but side wall of crystal does glow. I wanted to understand what is the relationship between emissivity ...
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What is the difference between thermal and coherent radiation?

Radiation, as far as I understand, is the transfer of energy through electromagnetic waves. The energy emitted from a hot body is known as thermal radiation. However, the accepted answer in this stack ...
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A question regarding the derivation of the equation for the Planck black body radiation distribution

There is something that does not make sense to me regarding a derivation that I am reading of the Planck distribution for black body radiation emitted from a cavity. It treats the photons as particles,...
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How does the black body photon distribution assume no reflections?

I understand that the definition of a black body is that it is a ‘perfect absorber and emitter with no reflections’ and while I see these properties being used to derive results in thermodynamics, ...
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Do bolometers measure all posible wavelengths of EM radiation? Is this even possible?

What is the closest thing we have to a 'black-body' bolometer? What are the best absorptive bolometers made of?
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Considering the history of microwave Background radiation, was it ever in visible range? When will it fall to be able to heat water like in our oven?

Please do provide assurance of what formula you used and that you did the calculations for the answer. Thanks. My back of letter calculation says that CMBR was never in visible range since it became ...
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Specular Reflection of IR Light

I have started to use thermal imaging cameras more and more and realized that I can see the reflection of my self bouncing off of a glass window through the IR camera. When viewing it on my monitor, I ...
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Black Body in empty space

Let's say we have a body of spherical shape with a temperature T > 0 Kelvin for our purposes. It's supposedly in empty space. Completely empty space, no other radiation, no far away stars nothing. ...
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How much near, mid, and far infrared radiation does a human absorb sitting one meter away from a campfire?

Here's my attempt to answer this. I'm not sure if I used the results from the blackbody radiation calculator correctly. The motivation for this question is to compare the amount of infrared radiation ...
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Intuition behind Johnson-Nyquist noise?

I've been doing a lot of reading on Johnson-Nyquist (thermal) noise lately, and sadly much of the material I've come across either looks at things from a very engineering prospective (that is, little ...
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Charged Black Hole: Self-Discharge

Palti writes in his introduction to the swampland (page 39) that a charged black hole (BH) could discharge itself, when the Hawking temperature $$ T_H=\frac{R_+-R_-}{4\pi R_+^2} $$ ($R_+$ and $R_-$ ...
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Where did this derivative term come from in this derivation? [closed]

I am stuck in the way this textbook shows how Planck arrived in the conclusion that the energy of a particle must be discrete. I have done the following, assuming that the energy $\Delta E$ is ...
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Blackbody radiation and photons in a cavity

Why is a consideration of a cavity filled with photons used to derive Planck's law? In other words, how is this cavity related to a blackbody of temperature T emitting radiation due to thermal/...

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