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2 answers
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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 ...
CP of Physics 's user avatar
0 votes
1 answer
60 views

Heating of a metallic 'gray body' by radiation

I am interested in a realistic model for calculating the heating of a metallic body by solar radiation. Assumption (0) is vacuum, so neither conduction nor convection. Assumption (1) is integration ...
TomS's user avatar
  • 947
2 votes
1 answer
142 views

Is a layer of gas with sufficiently large optical thickness really radiating as a black body?

Can a parcel of gas with large value of optical thickness really radiate like a black body? I have in mind a simple (most likely oversimplified) model which yields $$I_\nu = I_\nu(0) e^{-\tau\nu} + I_\...
MichaelW's user avatar
  • 1,391
9 votes
4 answers
2k views

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 ...
MichaelW's user avatar
  • 1,391
7 votes
1 answer
887 views

Could the solar shield on the James Webb telescope have been pitch black or does it need to be highly reflective?

When I look at pictures of the sun shield on the James Webb Space telescope (JWST), I see something that looks highly reflective (and hence must have a very low emissivity). My intuition tells me that ...
SalahTheGoat's user avatar
  • 1,599
0 votes
1 answer
63 views

Doubt regarding rate of loss of heat due to radiation

My doubt is with regards to radiation emitted by a body and how we define the rate of loss/gain of heat due to the same. For my question, let us consider a body of: * surface area A ...
anotherhyooman's user avatar
1 vote
0 answers
29 views

Absorption spectra Oxygen vs temperature

Whilst recently commissioning a spectroscopic Tunable diode laser "Oxygen Analyser" on a waste-to-energy plant, I wondered how the Analyser (which fires a narrow wavelength laser across the ...
scott hookway's user avatar
1 vote
2 answers
507 views

Absorption vs emission not in thermodynamic equilibrium

I have read that good absorbers are good emitters. The argument goes that at thermodynamic equilibrium, the amount of radiation a body emits must be the same as it absorbs, otherwise the body will ...
Yevgeniy P's user avatar
1 vote
1 answer
570 views

Thermal radiation vs emission spectra of gases

I am a high school student and I am very confused in absorption and emission spectrum of gases, for e,g take hydrogen at room temperature for simplicity, so that we can talk in terms of Bohr's model ...
Arun Bhardwaj's user avatar
1 vote
2 answers
933 views

If black is the best absorber and radiator, why does it get hot?

I can guess that it’s emission peaks at a higher temperature than white. So when a light is turned onto a black and white piece of paper, the initial condition is not at equilibrium. As black absorbs ...
aquagremlin's user avatar
  • 1,729
0 votes
0 answers
51 views

Black hole absorbing photons?

If a black hole has a radius that is not that much smaller than the wavelength of light emitted by the sun, and is at the same temperature, shouldn't it be able to absorb photons as well as emit them? ...
AWiltzer's user avatar
0 votes
1 answer
174 views

Are these statements about different emissivities correct?

I'd like to verify if my concluded statements regarding different emissivities are correct or not. And if not, why. 1 Two objects, one with an emissivity of 1 (object A) and one with a very low ...
Phy's user avatar
  • 471
1 vote
1 answer
522 views

Why do high altitudes have larger diurnal temperature variation than lower altitudes?

It seems like the lack of atmosphere should not be playing a role in the diurnal temperature variation because that's what makes it colder. Mountains are not that dry, usually.
Jake Borene's user avatar