All Questions
Tagged with blackbody or thermal-radiation
1,722 questions
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Planck's Law frequency vs wavelength [duplicate]
I was plotting planck's law today and attempted two different implementations. I took both from this table on Wikipedia. They are:
In terms of wavelength:
$$
B_{\lambda }(\lambda ,T)={\frac {2hc^{...
- 203
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46
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How can I calculate the temperature at a distance $r$ from a neutron star?
I am looking for an equation that can take into account the strong magnetic fields of neutron stars and the affect they have on the radiation of heat. So far, I have been unable to find anything more ...
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Is there a function for how hot an object would have to be to compensate for the drop off after SSR is exceeded using a microbolometer?
I am wondering if there is a clean formula for how much hotter a given object would have to be to register at the same temperature after the spot-size ratio has been diminished for a given thermal ...
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Why is there a colour difference between stars and hot objects since they are both black-body radiators? [duplicate]
I have a question about the color of a black-body radiator that has been baffling me for a few years now, and I haven't been able to find a satisfactory answer. So, we know that a star's color depends ...
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Event Horizon vs. "Interior" of a black hole from perspective of distant oberserver
From the perspective of a faraway observer watching an object fall into a black hole, they will see the object approach the black hole's horizon at a slower and slower rate until eventually "...
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488
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What is the purpose for the blackbody radiation graph to be graphed using the below parameters?
If you observe the above graph, for y axis, "intensity per wavelength" is used as the parameter. I am aware we use "per wavelength" because it is hard to measure and graph isolated ...
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3
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55
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Pair of polarizers as a black body
Let a system be composed of two perfect linear polarizers, where the axis of the first one is perpendicular to the axis of the other, so they do not let any light pass through the second polarizer. ...
- 1,870
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Experiments carried out to determine the energy of photons of different wavelengths
I imagine it would be very difficult to experimentally determine the energy per photon of monochromatic EM radiation. Since every material reflects/absorbs differently at different wavelengths, I can ...
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Does Stefan's law give the intensity of all the electromagnetic radiation or just infrared radiation?
The Stefan–Boltzmann law, also known as Stefan's law, describes the intensity of the thermal radiation emitted by matter in terms of that matter's temperature. (WIKIPEDIA)
The Stefan-Boltzmann Law ...
4
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Physics behind gold+silver space blanket placement
In the first aid and mountaneering, "space blanket" or "astro foil" is used for protection against cold enviroment or as first aid after burns. There are many different versions of ...
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4
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How is a black body 'radiating' light different from 'reflecting' it?
Light falls on a black body and its oscillating electric field causes the electrons to start vibrating, thus gaining kinetic energy which contributes to rise in temperature and, simultaneously, the ...
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Volume dependence on quantization of the electromagnetic field
I'm studying a non-additive formalism where volume dependencies play a crucial role, and I am particularly interested in applying these models to electromagnetic radiation, which has a well-defined ...
- 147
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45
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Can nonimaging optics concentrate heat to a higher temperature than the source? [closed]
Practical goal:
I'd like to concentrate the low-grade heat from the back of solar panels, which I've measured is in the range of 20-50F greater than the ambient temperature throughout the day, and ...
- 21
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1
answer
71
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Lambertian emitter law contradicts black body radiation
I'm really worried about "Lambert's law": A blackbody emitter is supposed to be "Lambertian" but I know from blackbody radiation that its radiance L (W/m²/Sr) is independent of ...
- 1,391
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51
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Planck's law in terms of flux [duplicate]
The Stefan-Boltzmann law describes blackbody radiation in terms of radiant flux per unit area i.e. how much energy passes through a 2D surface per unit time. I find this very simple and I understand ...
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How to apply symmetries ($q$-points) in the Brilloun Zone on the transmission function of radiation heat transfer?
Basically, I want to find out the near field radiation transmission function between two graphene, set at Temperature $T_1$ and Temperature $T_2$. As you can see in the figure d is the separation ...
- 121
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3
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157
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Can small solid bodies emit radiation with wavelength larger than the body itself?
It's known that EM waves cannot pass the aperture with a diameter much smaller than the wavelength. How is it for an opposite situation? If, for example, a solid body is a $100 \ \text{nm}$ sphere, ...
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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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61
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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 ...
4
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5
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745
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Why does not Earth radiate more energy into the space while it gets warmer? [closed]
The earth is getting warmer. By Stephan's law, it should radiate more energy into the universe.
On the other hand, the energy from the sun is a constant. So how to achieve a balance?
- 2,165
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4
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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 ...
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101
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Derivation of the solar resource at Earth's surface
I am reading Jenny Nelson's The Physics of Solar Cells and am a bit confused with the derivation of the solar irradiance at the Earth's atmosphere she supplies. I'll outline it here, and I provide ...
- 1,261
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2k
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Do black holes convert 100% of their mass into energy via Hawking radiation?
And if so, does that mean that we do not need matter/antimatter annihilations for 100% mass to energy?
- 461
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279
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Wien's displacement law
Wien's displacement law states the following
$$\lambda_{max}\propto\frac{1}{T}$$
However, after learning about redshift and blueshift, it doesn't make sense to me why we can use this law. Supposedly, ...
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1
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86
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Why does Sun appear white at noon?
I have found several source citing the reason for the white apperance of sun is due to less scattering of light before sunlight reaches an observer on the Earth (as it is relatively closer to us at ...
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2
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120
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Differential form of Planck's Distribution Law interpretation
So I didn't encounter differentials that often until now, I was taught that the seperate parts of $dy/dx$ for example are not supposed to have any sort of independent existence - ok.
(Calculus, 4th ...
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3
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1k
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Is the spectrum of Hawking radiation identical to that of thermal radiation?
Hawking radiation is frequently described as a form of black-body radiation, the same as non-black hole objects emit, except with the black hole's temperature defined solely by its mass.
But is this ...
- 301
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36
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Underlying mechanism for black body radiation [duplicate]
I understand that every body of some temperature above the absolute zero radiates EM waves. When we heat it up enough it enters the visible spectrum and than proceeds to go upwards to higher and ...
- 904
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3
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66
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When can we expect a black-body radiation?
During the pandemic last few years, the Infrared thermometer is widely used to measure the body temperature.
However, the fundamental question or the question in the first place is, why do we expect ...
- 2,165
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Is it true a cloud of free electrons emits no radiation?
We all know that atomic matter emits blackbody radiation according to its temperature even if no electrons are excited within that matter, however as far as I know no mechanism exists for blackbody ...
- 630
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How can the air be warmer than both the floor and the glass ceiling of an aluminum foil-lined box in the sun?
I've constructed a styrofoam box, with the insides spray-painted black, on top of which I placed five layers of borosilicate glasses, air-gapped with a bit of cork. It is diagrammed below:
The box ...
- 1,277
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42
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How to estimate thermal conductivity of a sample?
There is a rectangular sample whose emissivity is known, and I want to estimate its thermal conductivity. A heater with a known heat flux is applied to one end of the sample and a fixed temperature is ...
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3
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3k
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Is a hotter heat source always necessary for an object to be heated?
Heat always moves from a high-temperature object to a low-temperature object. Therefore, a low-temperature object cannot become hotter than the relative high-temperature object from which it is ...
- 401
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How to build a reverse greenhouse?
So basically, I stumbled across this concept of radiative cooling. There are a couple of YouTubers who posted videos on this topic.
What I understood, was that in passive daylight radiative cooling, ...
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Radiation from a 2D mirrored box
I came across an elegant maths proof here1, which is related to "black body" radiation. It's a kind of "toy thought experiment".
A unit square has perfectly mirrored sides but ...
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56
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Stefan-Boltzmann's law independence of surface density
One thing about Stefan-Boltzmann's law didn't make sense to me is that the radiated power density is independent of the particle density of a surface. It seems like things have to cancel out amazingly ...
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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 ...
- 65k
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742
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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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2
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79
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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 ...
- 393
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Cooling properties of trees and thermal radiation
A question about trees, shading and thermal radiation: Will the shaded area under trees have lower average air temperature than adjacent non-shaded areas?
On the one hand, I've read that the real air ...
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1
answer
56
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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?
- 1,742
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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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1
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44
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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 ...
1
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0
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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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54
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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 ...