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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How does heat travel?

How does heat travel? I have two competing thoughts here!! Firstly some form of atomic/molecular process liberates a photon in the infrared region of the spectrum which is detected as heat by a ...
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1answer
151 views

Is the physics of $j = \rho v$ questionable? What are the consequences? [closed]

In electrodynamics you have the quantity $\mathbf{j}$, which represents density of current per surface area. It is often said that $\mathbf{j} = \rho\mathbf{v}$, for geometric reasons. When I make ...
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2answers
292 views

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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0answers
20 views

How to understand/derive Eq 5.5 in Geiner's Quantum Mechanics - An Introduction?

Geiner's Quantum Mechanics - An Introduction has an unnumbered equation above Eq. 6 in section 2.4 discussing density -- not sure if it is energy density -- of radiation: ... $$dE/dV = E/V =a ...
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0answers
30 views

What would happen to the Earth's atmosphere if all the solar radiation was in the extreme ultraviolet?

According to this, our Earth's atmosphere is completely opaque to radiation with wavelengths less than 100 nm as this radiation has enough energy to ionize the air. Since the surface temperature of ...
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6answers
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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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1answer
62 views

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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2answers
863 views

Is a black body sphere inside a black body shell hotter than the shell?

I am missing something very basic here. Let us assume the shell is at a fixed temperature T by a power generator. The sphere inside will radiate the same power it absorbs. The power radiated by the ...
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1answer
159 views

Plancks law of blackbody radiation

I am going to plot this curve with wave length: $$ I(f)df = \frac{2 \pi h}{ c^{2} } \cdot f^{3} \cdot \frac{1}{ e^{ \frac{h f}{kT}-1 } } df $$ converting: $$ I( \lambda )d \lambda = \frac{2 ...
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1answer
221 views

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 ...
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1answer
60 views

How can Wien's Displacement Law be 'changed' to a version for frequency?

Wien's Displacement Law stated that for a blackbody emitting radiation, $$\lambda_{max}=\dfrac{1}{T}$$ where $T$ is the temperature of the body and $\lambda_{max}$ is the maximum wavelength of ...
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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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2answers
177 views

Does quantum mechanics contradict macroscopic determinism?

I am wondering whether it is true to ask whether determinism is still completely viable at macroscopic scales given that the constituent particles behave according to QM when the dimensions get small ...
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1answer
40 views

A confusion on Radiations emitted from a body [duplicate]

Suppose an atom is accelerated...Now the protons in its nucleus and the electrons in the orbits are also accelerated...So will they emit the electromagnetic radiations? Basically here the electrons ...
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1answer
151 views

Hawking (blackbody) radiation, emission or absorption

A black body is classically defined as a perfect absorber of radiation. That seems to fit a black hole pretty well. The only remaining question, then, How do we know that Hawking radiation is emission ...
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1answer
75 views

Why is the graph of CMB/black-body radiation asymptotic?

Speaking of this graph of blackbody radiation, I see that the graph goes to 0 asymptotically: As we go to higher and higher frequencies, the energy of a single photon becomes increasingly high. ...
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3answers
1k views

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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3answers
208 views

Asymmetric heat conduction?

So I have this side-view drawing. Now I wonder, will such a multi-layer material have asymmetric heat conduction properties? Namely, because of radiative conduction, reflective aluminum surface ...
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1answer
90 views

Modeling a wine cooler heat loss to ambient

I'm trying to model the steady state heat loss to ambient, in W, for a wine cooler similar to the following: For the modeling, I will need the following variables/constants: $T_a$ [K]: Ambient ...
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3answers
272 views

How to combat the black-body temperature of an object?

I'm trying to model the temperature of a large spacecraft for a space colony simulation game I'm working on. In another question, I checked my calculations for the steady-state black-body temperature ...
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1answer
59 views

How does the physical motion of atom lead to photon emission?

It's known that what we call a temperature is in fact molecular motion at microscopic scale. But at which point the emission of photons happens due to this physical motion, so that we can talk about ...
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1answer
98 views

Estimating the effect of radiant barrier on the radiant and conductive heat transfer through a metal roof

The basic question is -- Will a radiant reflective coating be more effective applied to the upper surface or the lower surface of a metal? Case 1: Imagine a metal roof subject to solar heating. The ...
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2answers
89 views

How hot particles can get [duplicate]

One way in which an object is affected by temperature rise is that the wavelength of the radiation it emits is gets smaller and smaller. Another way of looking at it is that as an object gets hotter, ...
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2answers
92 views

Blackbody radiation through quantum mechanics perspective

While explaining black body radiation, the body is assumed as a cavity radiator and the radiations are due to the oscillating electrons. But we know that the electromagnetic radiation emitted is ...
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1answer
57 views

Rayleigh-Jeans Equation

According to my textbook, the power radiated of a small hole in a cavity (an ideal blackbody) is given by $$R = \frac{1}{4}cU$$ where $U$ is the total energy density per unit volume, $R$ is the ...
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2answers
147 views

Why black body radiation is all over the frequency range

I was studying black body radiation and how quantization of energy solves the problem of ultraviolet catastrophe. But I have a very fundamental doubt. A black body can be assumed as a cavity with a ...
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3answers
384 views

How do objects heat up?

If every body emits radiation at a given frequency and temperature exactly as well as it absorbs the same radiation, how do objects heat up?
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1answer
131 views

Why do metal surfaces reflect thermal signature?

I recently borrowed a thermal camera from a friend and I tried to observe my palm print over a reflective metallic plate. When I looked at it through the camera, it reflected my thermal signature too. ...
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2answers
126 views

Blackbody radiation Color

An ideal blackbody absorbs all incident radiation. Josef Stefan found that the intensity $R$ (power per unit area) radiated by an ideal blackbody is given by $$ R = \sigma T^4 $$ Q1) Since an ideal ...
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1answer
225 views

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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2answers
57 views

Thermometer reading

I quickly plunged a room-temperature thermometer into very hot water, the mercury level went down briefly before going up to a final reading. Why?
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3answers
179 views

How damaging is light? [closed]

On Surely You're Joking Mr Feynman, when talking about the Trinity test, the author states: the only thing that could really hurt your eyes (bright light can never hurt your eyes) is ultraviolet ...
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8answers
699 views

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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975 views

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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1answer
41 views

Index of refraction appearing in the radiative transfer equation

In this publication the Radiative Transfer Equation (RTE) (eq. (7)) contains the following emission term: $$n_i^2\kappa_{d,i}L_{b,i}(\mathbf{r})$$ where $n_i$ is the refractive index of component ...
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2answers
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How can a metal heated at 600° emit thermal photons at 4000°+?

Suppose we have a cube of metal inside a room at temperature 27°. If we heat the metal up to 600° using uniform radiation of that energy, no part of it should have higher T°, but nevertheless it will ...
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2answers
1k views

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 ...
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1answer
197 views

Maintaining local thermodynamic equilibrium (LTE) in radiating gas with a broad atomic transition line

Definitions / Background In LTE, Kirchoff's law for radiation holds: $$ \frac{j_{\nu}}{\alpha_{\nu}} = B_{\nu} (T) $$ where $j_{\nu}$ is the specific radiative emissivity, $\alpha_{\nu}$ is the ...
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6answers
395 views

Extracting heat energy without a heat engine

Is it possible to extract the molecular kinetic energy from a system directly (without the use of a heat engine / temperature gradient) and convert that to another form of energy, such as electricity, ...
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2answers
158 views

Physical reason why (hot) objects glow? [duplicate]

Every object at a non-zero temperature radiates light, i.e. it glows. (Is that called blackbody radiation?) What is the physical reason to this? Is it because more heat implies that the atoms ...
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2answers
340 views

Radiation emission and absorption

Any object can emit and absorb radiation and the power of emission can be represented by the Stefan-Boltzmann law: $$P=A\epsilon\sigma T^4$$ In many texts the net power radiated is the difference ...
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4answers
484 views

How does quantization solve UV catastrophe in black body radiation? What would happen if there was no Planck constant $h$?

Planck's Law is $$I(\nu,T)=\frac{8\pi\nu^3}{c^2}\cdot\frac{1}{e^{h\nu/kT}−1}$$ This solves the UV catastrophe. For higher frequencies, intensity goes to zero. It does so because of $e^\nu$ not ...
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3answers
941 views

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 ...
3
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1answer
270 views

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 took ...
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1answer
3k views

Absorption cross section and absorption coefficient

What is the absorption cross section, how is it measured? How to convert it to the absorption coefficient (measured in cm$^{-1}$)?
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0answers
151 views

Absorption coefficient from HITEMP or HITRAN [closed]

How to calculate the absorption coefficient (for radiation) from HITEMP or HITRAN databases? Or where can I find some tables or plots for the absorption coefficient?
3
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1answer
60 views

Frequency-averaged (gray) radiative transfer

The equation for radiative transfer is $$ \omega \cdot \nabla I = \kappa(B - I) $$ where $I$ is the intensity of radiation, $\omega$ is the ray direction, $\kappa$ the absorption coefficient, $B$ the ...
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1answer
55 views

Sun radiation in England

I used to live in Italy, and when it was sunny, well, as expected it was pretty hot. Both under the direct light of the sun through the atmosphere, and in the shade, where the temperature was of ...
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How badly could someone be injured by concentrated sunlight?

Recently-ish, I stumbled across an interesting short story (by way of Science Fiction & Fantasy Stack Exchange) where a soccer referee is apparently incinerated by concentrated sunlight. Where ...
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Why does my infrared thermometer say the sky is at -2 °C?

I just got myself an infrared thermometer. I wouldn't have been able to predict what temperature it would give me when pointing at the sky, but it turned out to be -2 °C the first time I measured, and ...