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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### How well does a cavity with a hole approximate a black body?

Cavity with a hole is a frequently cited approximation to a black body (more precisely, it is the hole that is the black body here): Is there a simple estimate of how well such a cavity approximates ...
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### Trouble understanding the classic approximation of a black body as a hole on a cavity

While studying the Rayleigh-Jeans attempt to explain the spectral energy distribution of black bodies I have trouble understanding the concept of a black body as a small hole on a cavity. We define a ...
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### Regarding the absorption property of a blackbody

Consider a blackbody of surface area $S_b$ and at temperature $T_b$. It is placed inside an evacuated chamber (to neglect all the effects of convection), with walls of chamber at temperature $T_c$ and ...
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### Is there a technology that can reveal original carving on stone relief work that has been defaced? [closed]

Our house has a plaque on the wall from when the building was first constructed: it includes a coat of arms of whoever funded the building and some text below. Allegedly, the relief was defaced during ...
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### Two interacting blackbodies (one inside another) - when will thermal equilibrium be attained? [closed]

As I understand it, an ideal blackbody absorbs (and subsequently starts emitting) all incoming radiation. In typical setups like determining a planet's temperature given its albedo and distance from a ...
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### Radiation energy and momentum relation [duplicate]

Why is $\rho = 3p$ for radiation? What is the intuition behind this? If we had only 2 spatial dimentions, would it be $\rho = 2p$? (I came across this relation while studying the state of the universe,...
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### Surely $\rm CO_2$ reflects incoming solar infrared radiation?

Is the greenhouse effect not cancelled out by the $\rm CO_2$ in the atmosphere reflecting solar infrared radiation back into space? It seems logical to me that, if $\rm CO_2$ reflects infrared shifted ...
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### Does gas have thermal radiation of continuous spectrum?

Related question: Thermal radiation of a nitrogen sphere What kind of spectrum will a sphere of a heated gas have? Will this spectrum be continuous or linear? Will it have continuous part?
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### Thermal spectrum of a warmer body in a colder room

Here are a few questions about heat that I've been wondering about. Suppose I had a piece of glass (just as an example) at room temperature, let's say $T_0 = 293$ K, and I moved it into a dark room ...
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### Does Kirchhoff's law of thermal radiation fail for optically thin sources?

A box made of any material with a small hole in it will give blackbody radiation. According to my textbook by ARNAB RAI CHOUDHURI, If you place an optically thick source of same temperature as the box ...
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### Spectral Irradiation Plots: Units/ Order of Magnitude

I am hoping to get unconfused about calculations of irradiation vs the many plots I have found online for calculated spectral irradiance from the sun assuming it is a Black Body. I have found that my ...
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### Why is a cavity with discrete modes necessary in the derivation of Planck's law?

In my readings, I have come across the concept that a finite cavity allows for the quantization of electromagnetic modes, leading to discrete energy levels. However, it's not clear to me why this step ...
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### Determine thermal emissivity of material

Is there a practical way of determining it without a lab setup? interested in Aluminum alloys ADC-10 and ADC-12
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### Is the radiation from closed boxes blackbody radiation irrespective of the material of the box?

Suppose I have a box made of silver at temperature T, in thermal equilibrium with the surroundings. Silver has very low absorptivity and hence it will have the same low emissivity to maintain thermal ...
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### Is the Virial Theorem dependent on the classical Equipartition Theorem?

The Wikipedia entry for the Virial Theorem states: "*The significance of the virial theorem is that it allows the average total kinetic energy to be calculated even for very complicated systems ....
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### What do we mean when we say the CMB has a temperature and how do we measure it?

I have read this: An object without any internal degrees of freedom, like a single photon, can't really have a temperature. But an ensemble of photons can have a temperature. If you put an ensemble ...
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### Should I hold a baby formula bottle to cool it down faster?

This is an interesting and somewhat surprising physics problem - holding a hot object in your hand will cool it down faster, even if the air around is colder. I guess that 90% of people would be ...
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### In a universe with no photons, will everything necessarily be at absolute zero temperature?

Imagine a universe governed by the same physical laws as ours, i.e., the same fundamental forces, with the only caveat that there are no photons, hence no electromagnetic radiation in this universe. ...
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### Why can't we see green color in the sky? [duplicate]

From sunrise to sunset we can see a variety of colors in the sky. For example, during morning the most dominant color is blue whereas during sunset orange,red,yellow and there shades are more dominant....
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### Wavelength and frequency of the peak of the black body radiation [duplicate]

Why does the product of the peak wavelength and the peak frequency of Planck's black body radiation not equal to the speed of light? $$λ_{\mathrm{max}}=\frac{hc}{4.97\mathrm{kT}},$$ while ν_{\mathrm{...
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### Is this an error in deriving the Rayleigh-Jeans law? Kunstatter and Das's Symmetry SR and QM Ultraviolet Catastrophy

My question regards G. Kunstatter and S. Das, A First Course on Symmetry, Special Relativity and Quantum Mechanics, Undergraduate Lecture Notes in Physics, https://doi.org/10.1007/978-3-030-92346-4\_8 ...
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### Why only two polarisation states of the EM wave in the Jeans cube are considered?

This question already has answers here and there. But none of them answers , why there are only two polarisation states for a mode in the Jeans cube ? I mean they only say that we take the two ...
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### Total blackbody spectrum from two objects at different temperatures

I was wondering what the intensity spectrum would look like when collecting the light emitted by two objects (same material properties, emissivity etc.) at two different temperatures. Or alternatively,...
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### Radiation power emmited by a material with two different temperatures [closed]

Let's consider a cylindrical sample of a solid material surrounded by air. From $0 \leq r \leq r_1$ the temperature of the material is $T_1$ and from $r_1 < r \leq R$, $T=T_0$ which is also the ...
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### Blackbody in the gaseous state [closed]

Let's say that the blackbody is made up of some weird, not yet discovered, atoms. The radiation described by the Planck's law is the one emitted from such atoms when they're packed together, forming ...
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### Blackbody Radiation Simulation Software [closed]

I am trying to model the effective radiating temperatures of various three-dimensional solids (Tetrahedron, cube, octahedron, etc.) given a specific incident radiation. Are there any (free) softwares ...
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### How do differently colored objects reach the same temperature if they are completely isolated together?

Imagine a perfectly reflective container, filled with vacuum and two bodies with different colors, i.e. they have different emissive and absorptive properties. The bodies don't touch each other or the ...
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### Why is my pyrgeometer calibrated as if it has $a >1$ emissivity?

I've acquired an SL-510-SS upward-looking pyrgeometer. In the manual it gives the formula for how to compute the downwelling infrared it picks up: Clearly the $k_2$ coefficient is the emissivity of ...
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### How to measure light intensity in a room?

Does anyone know how I would go about measuring the light intensity in a room? I'm not interested in knowing the lux reading, I would like to measure the $W/m^2$ due to thermal radiation in my ...
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### Thermal emissivity for cool pavements

I have a very basic question that I'm struggling to find a clear answer to as I do research about cool pavements. The literature says cool pavements should have high albedo, high thermal emissivity. ...
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### Why are planets cold?

Excuse me for me ignorance, I'm just fiddling around this question for quite a while. The question of course is not limited to planets. Boiling down to, why isn't the universe heated up by now to ...
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### How were luminous intensity changes for each EM frequency measured in the blackbody experiments of the late 19th c., which led to up to Planck’s Law?

If in today’s parlance it would be either ‘spectral density’ or ‘spectral radiance’ that was in fact measured then as opposed to ‘luminous intensity’ please feel free to clarify. Kindly note that my ...
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### Beam splitter using a lazer to do the splitting

My title may be misleading, and my question is definatly amateur. I would like to split a beam of light, with a second beam. I suspect this may not be possible (or only possible with large amounts of ...
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### Linearization of long wave radiation equation

I am basing my reduced order building model on the equations proposed in the article “A methodology for generating reduced-order models for large-scale buildings using the Krylov subspace method” ...
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### Does this system involving blackbody radiation violate the second law of thermodynamics?

Consider the surface of rotation shown in heavy lines in the accompanying sketch. It is made up of parts of the surfaces of two confocal ellipsoids of revolution, and that of a sphere. The inside ...
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