All Questions
Tagged with blackbody or thermal-radiation
303 questions
69
votes
8
answers
11k
views
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 ...
- 1,556
16
votes
5
answers
2k
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How does radiation become black-body radiation?
Textbooks treat black body radiation as radiation in thermal equilibrium with its environment (more specifically - with a black body): Planck's formula is essentially derived from the partition ...
- 65k
18
votes
1
answer
38k
views
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 \...
- 291
6
votes
2
answers
1k
views
Black body vs. Thermal radiation
I am seeking clarification of the following terms:
Black body radiation
Thermal radiation
Thermal light source
At the first glance, the first two are the same thing, and they are the radiation ...
- 65k
24
votes
3
answers
10k
views
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 ...
- 5,452
28
votes
7
answers
8k
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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 ...
- 1,918
12
votes
4
answers
2k
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How is light emitted by an incandescent lamp?
I am looking for better understanding of how light is produced in an incadescent lamp. More specifically: how is the kinetic energy of electrons converted to light?
Are we dealing with interband ...
- 65k
76
votes
5
answers
63k
views
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 ...
- 1,938
35
votes
5
answers
7k
views
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 ...
- 8,260
28
votes
3
answers
15k
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 ...
- 293
31
votes
1
answer
69k
views
Why is black the best emitter?
Why are emitters colored black better emitters than other colors? Why is white a worse emitter?
- 425
10
votes
2
answers
4k
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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 ...
- 279
21
votes
4
answers
10k
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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 ...
- 2,908
13
votes
3
answers
8k
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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 constant.
I ...
- 3,074
40
votes
4
answers
26k
views
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 sky at night white?
I suppose it could be due to ...
- 775
19
votes
5
answers
7k
views
Does fire emit black-body radiation?
Question: Can the radiation emitted by fire be approximated by black body spectrum?
It has been discussed in this community that black-body spectrum mostly serves as an approximation to actual spectra ...
- 65k
16
votes
5
answers
30k
views
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. :( ...
- 3,461
9
votes
4
answers
11k
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How can blackbody radiation be explained by quantization?
I don't understand why quantization makes a peak in the blackbody radiation curve (so there is no UV catastrophe) and the concept of a relationship between that peak and quantization.
When the ...
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7
votes
2
answers
3k
views
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 ...
19
votes
3
answers
15k
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 this ...
- 2,489
17
votes
5
answers
4k
views
Stefan–Boltzmann law applied to the human body
The average person consumes 2000 kcal a day, which is equal to ~100 W. Furthermore, if one uses the Stefan–Boltzmann law to calculate how much someone loses heat due to radiation, it can be seen that ...
- 365
16
votes
3
answers
3k
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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 ...
- 3,811
11
votes
2
answers
2k
views
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 ...
- 22.5k
10
votes
2
answers
3k
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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 ...
- 908
5
votes
1
answer
788
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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 ...
- 95
3
votes
2
answers
2k
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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 ...
17
votes
4
answers
10k
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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 ...
- 16.7k
16
votes
5
answers
5k
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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 ...
- 3,364
10
votes
2
answers
737
views
How does Hawking radiation emerge from the shadows?
Imagine you have a large black hole, onto which you drop a small object. This object could have any shape you like, but later on it will look like a thin sheet, so we could imagine it starts that way, ...
rob♦
- 94.2k
6
votes
2
answers
3k
views
Why is the sky blue and the sun yellow?
The blue color of light of the sky is due to Rayleigh scattering.
But the sun itself appears yellow in color whereas the scattered sunlight itself appears blue.
Why does this happen?
Should the sun ...
- 454
3
votes
3
answers
21k
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 ...
- 43
2
votes
3
answers
1k
views
Difference in thermal radiation between condensed matter and gases
In Quantum physics of atoms, molecules, solids, nuclei, and particles by Robert M. Eisberg and Robert Resnick, the authors write:
Matter in a condensed state (i.e., solid or liquid) emits a ...
92
votes
7
answers
28k
views
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?
...
- 1,657
33
votes
6
answers
15k
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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 ...
- 2,784
15
votes
8
answers
3k
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 ...
- 25.2k
11
votes
8
answers
24k
views
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 ...
- 2,565
10
votes
2
answers
964
views
Why does each mode of the electromagnetic field have two degrees of freedom?
In the derivation of the Rayleigh-Jeans law, using the Equipartition theorem the number of modes per unit frequency per unit volume is multiplied by $kT$, which implies each electromagnetic resonant ...
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9
votes
4
answers
11k
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Light formed by the sun?
This is an extract from the astrophysics chapter in my book:
Hydrogen atoms fuse to form helium. At the same time, lots of of gamma photons and neutrinos are produced. The photons take thousands ...
- 2,177
5
votes
2
answers
45k
views
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}$ ...
- 345
4
votes
1
answer
238
views
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 ...
- 1,999
3
votes
5
answers
619
views
How do photons reach thermal equilibrium with the walls of the blackbody cavity?
There's a lot that has been confusing to me about Planck's radiation law, and at this point I feel like I've got the hang of most of it. There is really only one last question that I can't resist ...
- 115
1
vote
1
answer
413
views
Why does the ground-state energy of the quantum harmonic oscillator result in ultraviolet catastrophe? [duplicate]
Background - This post is closely related to this question.
Planck considered that the electromagnetic waves inside a cavity are standing waves due to oscillating charges on the wall of the cavity. At ...
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21
votes
7
answers
5k
views
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 ...
- 737
20
votes
6
answers
7k
views
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 ...
- 34.9k
17
votes
5
answers
28k
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Why is sunlight spectrum continuous?
My teacher explained today that unlike the spectrum obtained by analyzing the sunlight, the spectra of atoms are not continuous.
I have a question about this - the sunlight is emitted by the atoms of ...
- 5,839
12
votes
1
answer
20k
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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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10
votes
2
answers
6k
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Historic derivation of Wien's law
Every book I've read, including a lot of websites, Wikipedia, etc, say that Wien derived this:
$$\rho_\nu(T)=\rho(\nu,T)=\nu^3f\left(\frac{\nu}{T}\right)$$
Being $\rho_v(T)$ the spectral enegy ...
- 1,443
8
votes
1
answer
5k
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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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7
votes
3
answers
5k
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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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5
votes
4
answers
10k
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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.
...
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