All Questions
39 questions
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Spectral energy density of black body radiation [closed]
$u(\nu,T)$ is known as the amount of energy emitted per unit frequency $\nu$ per unit volume $V$. It is the function of $\nu$ and $T$. In case we would like to know the amount of energy density at ...
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How is radiation emitted from a small hole in a box, if the hole is small enough to prevent radiation from escaping in the first place?
So I totally understand how a hole in a box works as a good approximation for a perfect absorber of all incident radiation (a black body). However, a black body is also a perfect emitter of radiation ...
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4
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1k
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Matter made of neutral charges does not radiate?
We know that any blackbody that has a certain non-zero temperature radiates. This happens due to the jiggling of charged particles (protons and electrons), and accelerated charges create ...
- 111
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48
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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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1
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173
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Thrid law of thermodynamics and quantum mechanics
Third law of thermodynamics says that it is impossible to reach absolute zero temperature in finite numbers of operations. According to quantum mechanics, every system has energy levels and ground ...
- 119
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1
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79
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Planck's assumption on blackbody radiation
I have just started basic quantum mechanics and have come across this expression for energy density of blackbody radiations; Planck's formulation.
$$ u_{\nu} \ d\nu = g(\nu) \ \langle E \rangle \ d\nu
...
- 129
3
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3
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929
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Why does a blackbody cavity with a hole emit black body radiation?
Why could we use nearly every material in a cavity with a hole to absorb/ emit every wavelength?
If the material doesn't have the exact energy gap, g, for example, it couldn't absorb a photon with ...
- 153
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1
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74
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Distribution of Electric oscillator's in Boltzmann's theory
While I was reading the book "In Search of Schrodinger's Cat" I found an interesting excerpt on how Max Planck used Boltzmann's statistical equations to solve the Blackbody radiation problem....
- 41
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61
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Does the radiation spectrum of bodies depend on it composition or not?
In the second edition of Quantum Physics by Robert Resnick and Robert Eisberg, they write
"Matter in a condensed state (i.e., solid or liquid) emits a continuous spectrum of radiation. The ...
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4
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2k
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Why can Einstein coefficients be derived based on thermodynamically equilibrium relations when they are basically intrinsic?
Einstein coefficients for emission and absorption ($A_{21}$, $B_{12}$, $B_{21}$) are often derived from a consideration of thermodynamic equilibrium using Boltzmann statistics and comparison with ...
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3
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108
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Question on black body radiation
In black body radiation at thermal equilibrium, the absorptivity is equal to emissivity (to conserve energy). Is it fair to say that all of the photons falling on the body are absorbed and emitted? If ...
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3
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441
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Why is the black body radiation independent of composition and incident radiation?
There have been questions similar to this, but most of them do not explain the mechanism responsible for the phenomena but instead explain through contradiction of second law of thermodynamics, for ...
- 659
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3
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430
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Why using a sum instead of an integral solves the Ultraviolet catastrophe?
Im reading about how using a sum instead of an integral when deriving the Rayleigh-Jeans law (in the equipartion part) solves the ultraviolet catastrophe. basically doing the following replacement:
$$\...
- 97
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1
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100
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Apparent radiation pressure paradox
Imagine a plate beeing irradiated from all sides with a specific wavelength. One side of the plate is smoth and reflects most of the incoming radiation, the other side is covered in grooves with the ...
- 119
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0
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81
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Can one derive Planck's law by considering a more "natural" setting?
I have been wondering about this for a while. I've recently become a bit dubious regarding the usual derivation cited in textbooks of Planck's law (and its inaccurate predecessor, the Rayleigh-Jeans ...
- 20.8k
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Planck's derivation of Blackbody Radiation
I have been studying quantum mechanics(blackbody radiation basically) and came up with an equation in Planck's derivation(where he had to assume that the oscillators in the cavity walls were limited ...
- 172
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2
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630
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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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238
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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 ...
- 1,999
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1
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1k
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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 ...
- 651
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0
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Information loss problem if almost perfect black body
Assume a almost perfect black body at temperature zero and consider pointing a signal, let's say a laser beam (pure state), on this body. What happens with the information of the beam?
My thoughts go ...
- 173
3
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3
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251
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Why does $hf$ in Planck's formula imply quantization?
It can be a very naive question, but I came across a textbook saying:
$hf$ suggests that the energies of the atomic oscillators in the cavity wall are quantized.
But I don't get the why. I already ...
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0
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105
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probability of emission vs absorption in thermal radiation
I am trying to make sense of the thermal radiation emitted by a gas.
The radiative transfer equation is $$\frac{dI}{dx} = \epsilon - \kappa I,$$ where I is the intensity, $\epsilon$ is the ...
- 323
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2
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678
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Two questions about the standing waves in a black body
I am currently reading a derivation of Rayleigh-Jeans law for cavity radiation from Eisberg and Resnick 1 . The authors derive the law by considering a cavity with metallic walls. In the book, the ...
- 1,314
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4k
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Is the quantum harmonic oscillator energy $E = n\hbar\omega$ or $E = (n + 1/2)\hbar\omega$? (Feynman Lectures)
Please, read the whole question. I've discussed a few contradictions and so far have not found an explanation for them.
I was reading The Feynman Lectures on Physics (vol. 1), the part where he talks ...
- 1,263
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3
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588
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Intuitive understanding of the derivation of the Rayleigh-Jeans law
I know the Rayleigh-Jeans law and how the formula predicts UV catastrophe. Without getting into the exact derivation, I am trying to get some intuitive understanding of it by using some of the broad ...
- 1,471
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382
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Feynman Lectures Vol I 41-2: why can $\omega$ replace $\omega_{0}$ in deriving Rayleigh’s law?
NB: I understand that the model presented here is the "failing" classical model. I'm just trying to understand the formal reasoning of the model.
In The Feynman Lectures on Physics Vol. I ...
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1
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311
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Spectrum from cavity radiators
A hole in a cavity kept at a constant temperature is supposed to approximate the behaviour of a black body. If an external heat source is keeping the body at a constant temperature wouldn't the heat ...
- 150
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0
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58
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Transitions coupled to different baths in quantum thermodynamics
I don't understand the general assumptions made during the separation of a single environment of the open system represented by the energy levels into two baths at two temperatures in the diagram ...
- 223
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136
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What is the meaning of "radiative distribution of entropy"?
I was reading about black body radiation on Wikipedia. In the explanation section, it says:
"All normal (baryonic) matter emits electromagnetic radiation when it
has a temperature above absolute ...
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1
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2k
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Black body radiation at atomic level [duplicate]
I need help in black body radiation. I know quantitative equations of black body radiation. what I need to know is what is happening inside a black body at atomic level. What I mean is, if light ...
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Confusing derivations of Planck's Law in different books
I was studying the derivation of Planck's Law. But I found confusing texts in different books.
In the book "Quantum Physics of Atoms" by Eisberg and Resnick, it states that Planck considered the ...
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2
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422
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Understanding thermal radiation in a conductor, gas and insulator
Context:
I was in my bliss of ignorance and happiness when I was taught that quantum mechanics was about nice discrete values of energies. Now I am introduced the idea of Fermi Energy in a block of ...
- 103
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2
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969
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Is this conceptualization of blackbody radiation logical/correct?
From what I understand, a blackbody is a body which does not emit radiation as a result of atomic excitation/relaxation but rather solely due to the kinetic energy of its particles due to interactions ...
- 428
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1
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2k
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Dr. Pierre-Marie Robitaille: On the Validity of Kirchhoff's Law
Lately I've been researching about the black-body spectrum and the historical development of Planck's Law. I mainly wanted to understand a little bit more why many different objects (Stars, Hot Metals)...
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1
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884
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Planck's postulate for oscillators or for light?
I know that Planck originally postulated that the energy of an oscillator in a black body was quantised to $E=nh\nu$ but did he know at the time that this meant the energy of light was also quantised (...
2
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2
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2k
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Blackbody radiation and emissive power
According to blackbody radiation theory, and thanks to Planck, we now know that there is a energy density, $u(\lambda,T)$ [$J/m^3$], associated with a certain wavelength at a particular temperature. ...
- 111
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1
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2k
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What's the relationship between the energy density of a black-body and its radiant exitance?
Through a bit calculation we can derive that in a cavity, the energy density $$u(f,T)=\overline{E(f)}\times G(f)=\frac{8\pi h}{c^3}\frac{f^3}{e^{h\nu /kT}-1}$$
If we take the integral over all ...
- 1,188
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1
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193
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Landauer's principle vs Wien's displacement law
Can we argue based on Landauer's principle that if one bit information is changed inside a blackbody, the total radiated energy should be at least or in order of kTln2? If it is so, can we also argue ...
- 238
1
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1
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167
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Landauer's principle vs Rayleigh–Jeans law
Can we argue based on Landauer's principle that if one bit information is changed inside a blackbody, the total radiated energy should be at least or in order of $kT\ln2$? If it is so, can we also ...
- 238