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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Planck theory of discrete energy of photons and electromagnetic spectrum

Planck said that "energy of photon is quantized. That means energy of a photon cannot be any random value but a certain values only". If this is true how electromagnetic spectrum is continuous. For ...
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Black body radiation and line spectra

Firstly, Blackbody radiation and line spectra are both a phenomenon of energy emitted(EM), but why is the blackbody radiation spectrum continuous while line spectra is not continuous? Secondly, ...
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Could a foggy double-paned window be amplifying UV rays?

Ever since my living room double-pane window got foggy, I've noticed that I feel very uncomfortable/sick when it is a very sunny day (similar to the way I feel when I've been out for too long in the ...
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Why doesn't hot charcoal glow blue?

I was learning about black body radiation and the explanations given by Max Planck and Albert Einstein when a thought crossed my mind. When we heat an iron piece, its color changes gradually from ...
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The equilibrium era and energy density

In Weinberg's 'Cosmology', at page 109, he presents an era "of equilibrium" when "the temperature and density were sufficiently high so that radiation and matter were in thermal equilibrium. Then, ...
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What is the average energy of visible light photons, of what we perceive to be white light? (i.e. visible spectrum of 5500K radiation)

Visible light is anything between 1.65eV and 3.10eV, so the answer is somewhere between these values. Naively we could average this to 2.375 eV, but that would not be actual average of full spectrum ...
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Can radiators cool down using only back body radiation?

If you could produce a very thin tube, say less than the thickness of a human hair, designed to carry a hot liquid inside, the diameter is small so that the liquid would have a high surface area to ...
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What processes permit (presumed) emission by, but prohibit absorption of, IR photons by non-greenhouse gas molecules?

It is said that because of bond-structures / possible vibrational states, non-greenhouse-gases / homonuclear diatomics cannot absorb IR. Presumably there is no question that all gases, being matter, ...
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Frequency and wavelength of thermal radiation [duplicate]

Why for a given temperature these forms of Wien's law implies a different maximal wavelength?
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Principle of a pyrometer

How does a pyrometer work? Especially how do I ensure that the temperature of a single spot is measured (where the laser points at)? Here is a scheme of the structure but I guess it's not really ...
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Black body radiation curve

See images about black-body radiation. How does Planck's quantum theory explain the low intensity of radiation for high frequencies? I.e why does the black body curve become lower at the high ...
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Is it possible for a radiant heat transfer to create higher temperature in the sink compared to the source?

Assume we concentrate the rays of the sun onto an object (in the vacuum of space) that has very low emissivity - The energy coming in is higher than the amount being emitted as radiation - hence it ...
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Why do thermal cameras work at infrared radiation and not at visible one?

From the Wien's law I have computed the Spectral Irradiance of a black body at 1000°C. From different thermal cameras manufacturers web sites, a lot of Thermal Cameras work in the range 6-14 $\mu m$....
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Do initially heat(radiation) shield has temperature between the high temperature end and low temperature end?

Suppose two constant temperature (T1) and( T2 ) (T1>T2) large plane surfaces are there , in order to reduce heat loss we provide a heat shield in between them , i know at steady state there exists a ...
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Wien's Displacement Law (Graph) [duplicate]

Why do two different graphs (i.e at two different temperatures) of Wien's Displacement Law do not intersect at any point? (Please try to give answer using basic (+2 or UG level) physics.)
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Silica aerogel - reflectivity and absorption

Silica aerogels are 97 percent porous, meaning light moves through the material but the interconnecting nanolayers of silicon dioxide trap infrared radiation and greatly slow the conduction of heat. A ...
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Inconsistency in two equations for Planck law given by professor

I know there are a variety of questions about this topic but I have been searching the internet for a long time and couldn't find a proper answer. The question is simple: I saw the following formula ...
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Is the integral of the spectral irradiance graph equal to the total energy absorbed?

This image is found on wikipedia: My question is essentially: how can I read from this graph the actual total energy absorbed by the earth? Is it true that the total red area is equal to the solar ...
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Temperature of an opaque body with known reflectance

Assume we have an opaque body, irradiated from all sides with a spectral irradiance $E(\lambda)$ [W/m2]. Furthermore, we know its reflectance $R(\lambda)$. (It's in vacuum, and in no thermal contact ...
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Doubt about Entropy of solar radiation on Earth

This paper $[1]$ gives the following statement: On Earth, the low-entropy energy is provided by solar radiation and the high-entropy energy is disposed as infrared radiation emitted into the cold ...
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Equilibrium temperature of a black body when the volume of the black body is changed

If a black-body is at an equilibrium temperature T of a certain volume. If we increase size of blackbody does the equilibrium temperature of the blackbody will change?
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Proportion of heat or energy passing through a window by radiation vs conduction

I'm trying to understand how modern window films or coating (so-called "Low-E" coatings) can improve the "R" value of the window just by reflecting internal room radiant energy (presumably long-wave ...
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How long does it take for an electron to reach equilibrium with blackbody radiation?

While teaching a course on electrodynamics, I thought of an interesting question that I think deserves some attention. Consider an ensemble of electrons all with momentum $\hbar \mathbf{k}$ ...
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Does an object storing more internal energy emit more thermal radiation?

There are two objects of the same material, similar geometry but different dimensions and same temperature. The larger object is supposed to contain more quantity of heat because it has larger volume. ...
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Is there a $\pi$ in Planck's Radiation Law?

In Sear and Zemansky's University Physics with Modern Physics 13th edition and other sources like this post, it says the formula for Planck's radiation is given by: $$I(\lambda)=\frac{2\pi hc^{2}}{\...
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What is “power per unit frequency” in black body radiation? [duplicate]

What is the meaning of power radiated by a black body per unit frequency? If you have a black body with a frequency filter around it set to 530nm and you calculate the energy radiated in 1 second you ...
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What does 'per unit frequency' refer to?

We were discussing Kirchoff's theorem on blackbody radiation in class today and the equation was: $$e_{f}= J(f,T)$$ $e_f$ was defined as power per unit area per unit frequency. What does per unit ...
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Why is the spectrum of a star pretty much continuous? [duplicate]

I was reading about the development of the quantum theory when I got to the explanation for spectral lines. It's a topic that I've revisited many times but I came up with a question. I know that in ...
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How do I calculate the relative number of photons per wavelength emmited by the sun?

Assume I generate $n$ photons emmited by the sun. They have wavelengths between 390 and 750 nanometers. How do I find out the number of photons that correspond to each wavelength: How many of my $n$ ...
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Do a collection of black body emitters emit radiation in synchronization with each other?

Just read that all bodies at temperature T>0K emit radiation whose spectral characteristics depend on the temperature of the body. I am thinking of a macroscopic body as a collection of several tiny ...
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What is the significance of an ideal blackbody? [closed]

Why the concept of an ideal blackbody had been introduced? What was its need or its significance?
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Blackbody curves at low temperature

There is already a post here concerning a blackbody curve in visible light. The answer was helpful in that it stated that an object emitting visible light would emit some higher energy photons in x-...
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thermal equilibrium in deep space

Imagine an object- say, a sooty asteroid- in deep space, far enough from any star to receive only negligible radiation from it. We make it sooty so as to make it a good absorber of radiant energy and ...
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What are all the types of radiation emitting from the Earth or an arbitrary mass in space?

I'm interested in what forces collectively act upon a body in space (obviously radially, so they usually cancel) due to pressures caused by emitted radiation (of any type, strength, "size"). Can you ...
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How Can the Number of Degrees of Freedom per Frequency in the Rayleigh-Jean formula be Irrational, or are they not?

I've read that the Rayleigh-Jean formula for the energy density per frequency of blackbody radiation can be expressed as $u(ν,T) = nkT$ where k is Boltzmann's constant, T is the temperature, and n is ...
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What is the radiation field of a black body with temperature $T$ translating at speed $v$?

Suppose I have a spherical black body at temperature $T$ with radius $R$. When stationary in a vacuum at zero Kelvin, it produces an incoherent radiation field according to Planck's law $$ B_\nu = \...
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Conduction, convection, radiation: Does evaporation count as one of those?

The forms of heat transfer are traditionally described as conduction, convection, and radiation. Is evaporation (or any other change of state) counted as one of those forms? Or is it considered its ...
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Nature of blackbody oscillators [duplicate]

I know that thermally excited materials "act as atomic oscillators" which can oscillate in a continuous range of frequencies, producing light in said frequencies. But in my searches, the nature of the ...
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Does the iron melt? Radiative-heat transfer

A molten metal (iron type A) is in vacuum (no air) at a constant temperature ($T_m$) equal to 1500 C (equal to its melting temperature) with emissivity $\epsilon_m$. At a distance of 10 cm there is a ...
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If I stood next to a piece of metal heated to a million degrees, but in a perfect vacuum, would I feel hot?

A friend of mine told me that if you were to stand beside plate of metal that is millions of degrees hot, inside a 100% vacuum, you would not feel its heat. Is this true? I understand the reasoning ...
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How hot can the surface of a planet be?

I just read this article https://edition.cnn.com/2018/07/02/world/newborn-planet-image-study/index.html and noticed that the astronomers estimates the surface temperature of the planet to 1000 °C. ...
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Waves in Box are Black Bodies?

From what I understood, a Blackbody is a perfect absorber, and hence also a perfect emitter since it is in thermal equilibrium. One example of a blackbody is a box with interior walls painted in black....
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How does the photon energy formula $E=h\nu$ mean the quantum nature of energy?

The frequency v can be less than 1. So the energy is starting from zero, and is continuous. The black body radiation is also continuous spectrum given an arbitrary temperature T.
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Deriving intensity of light as a function of frequency

Page 4 of the textbook Quantum Field Theory and the Standard Model by Schwartz says the following: The incompatibility of observations with the classical prediction led Planck to postulate that the ...
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Do metallic meshes work in practice as thermal radiation shields?

The other day I was thinking about the wire mesh cover used in microwaves to keep the microwave radiation inside cooking your food. The idea is that the spacing of the mesh is much smaller than the ...
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Does water vapour also emit radiation when rising into the air before condensing?

There are topics on how water vapour absorbs radiation ("How does carbon dioxide or water vapour absorb thermal infra red radiation from the sun?"), but what I am looking for is can water vapour also ...
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How many photons are emitted by a lightning strike?

As the title suggests, I'm curious to know, approximately how many photons are emitted in a single lightning strike?
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Deriving Planck's Constant from Wien's Displacement Law

So I'm reading an introductory book on Quantum Theory (David Park, 3rd ed.) and I am having trouble with the following question: "According to Wien's displacement law, the wavelength $\lambda_m$ at ...
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Effect of anodisation dye colour on absorptivity-to-emissivity ratio of aluminium

Anodised aluminium is a common material used in space, and it is commonly dyed black as this gives it an absorptivity-to-emissivity (a/e) ratio close to unity. This page says black anodic films have ...
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Can we concentrate the “cold of the sky” with a parabolic dish to cool things down during the day?

It's well known that we can focus sunlight with a parabolic dish to collect heat. But if we look at the thermal images of the sky we can see that the clear blue sky is quite cool. So by turning the ...