Talk:Thermal radiation

message: In the "Surface Effects" section in Overview it states the following: Personalized cooling technology is another example of an application where optical spectral selectivity can be beneficial. Conventional personal cooling is typically achieved through heat conduction and convection. However, the human body is a very efficient emitter of infrared radiation, which provides an additional cooling mechanism. 
Two problems 
(1)I believe this is false  
(2)No reference is provided. 
(2) doesn't need any explanation.  I will now explain why I believe this to be false. Consider thermal radiation and conduction from a human body. If the human metabolizes at M=150 watts (about 3000 kcal/day) then their radiative flux is about M/A where A ~=2m^2 is the surface area which admittedly has considerable uncertainty. This heat flux then is about 75W/m^2 and has to be carried away by radiation and conduction. Radiation is by far the dominant effect. I'm going to ignore the emissivity because it is very close to 1. The radiative flux, phi_rad is given by phi_rad = sigma (Ts^4 - Tw^4) and the conductive flux, phi_cond=  kcond(Ts-Tw)/L. Here Ts is the temperature of human skin, Ts=310K, and Tw is the temp of the surrounding walls, say Tw=295K, with L the distance from the person in the center of the room to the walls, say L=5m and kcond = .026W/m-K is the thermal conductivity of air. phi_rad ~= 76W/m^2 is two or three orders of magnitude larger than phi_cond ~=0.1W/m^2, so in what sense is "personal cooling" "typically achieved through heat conduction"? 
I think the main mechanisms by which heat leaves a resting body are (1)radiation, (2)exhalation of water vapor, and (3)sweat and evaporative cooling during exertion. 
I don't see any way to get significant amounts of heat transport from conduction/convection as compared to these other mechanisms. Presumably this is discussed in a text book somewhere?    — Preceding unsigned comment added by 76.113.29.12 (talk) 00:50, 13 November 2021 (UTC)
thread_title: Overview

message: The article states that the color of an object has little effect in its thermal radiation at IR wavelengths at ordinary temperatures.  This is clearly untrue.  Every auto mechanic knows that black-painted auto radiators cool their contents better than light colored ones.  I had a frying pan which has a dark nonstick surface on one side and shiny aluminum on the other.  If it is heated to maybe 100 or 150C and held vertically at a horizontal distance from my face, no heat is felt when the shiny side is facing me, when the dark side is facing me the heat can be felt instantly from a meter away.  
I came here to learn more about this effect and how to quantify it, only to read a claim that the effect does not exist at all!
Hopefully someone (I'm not a physicist) can fix this part of the article.  — Preceding unsigned comment added by 199.115.12.254 (talk) 22:19, 20 September 2016 (UTC)
replies: text: Several years ago, a mechanic told me that every mechanic knows that's myth.  — Preceding unsigned comment added by 92.67.227.181 (talk) 14:24, 4 July 2022 (UTC)
thread_title: Object Color and Thermal Radiation at IR Wavelengths

message: I've reworked the article, when I found many factual errors and other nonsense. Please post a comment and compare with the old version, as you see fit. Again, refrain from correcting as per old version - it was full of factual errors. 
--PoorLeno 22:43, 7 April 2006 (UTC)
replies: text: Try harder.  Radiators heat by all three mechanisms.  If you're close to the radiator then you can feel the IR radiation.  You know it's radiation because you can shield yourself from it with your hand or a piece of reflective material.  If you're on the opposite side of the room then most of the heat you receive will be from convection.  If you touch the radiator then it's conduction. --Heron 19:54, 21 June 2006 (UTC)
replies: text: Hm, perhaps it was a bad example. I was visualizing a radiator in vaccum. I guess a better example would be heat from a fireplace, where you feel the infrared radiation as you come close to it. But at the same time, I wanted to illustrate that the surface itself is radiating. PoorLeno 10:58, 18 December 2006 (UTC)
replies: text: And what about the heat that the air itself conducts? How do you discern between this and radiation? James Callahan 21:35, 23 April 2007 (UTC)
replies: text:  Put a piece of thin plastic like a trash bag between you and the heat source. Tarchon (talk) 20:46, 17 August 2009 (UTC)
thread_title: Reworked

message: I was looking for information on how color changes the radiative spectrum.  Can we get some examples of emissivity correction factor functions, perhaps with a chart? -- Beland (talk) 19:44, 21 November 2007 (UTC)
thread_title:  Non-black bodies 

message: it could stand to be redone such that the colors of the lines corresponded to the peak wavelength. That is, going from blueish, to greenish, to reddish, to black. Hantavirus (talk) 15:25, 12 June 2009 (UTC)
replies: text: agreed, as is I think this could confuse a lot of people--Xris0 (talk) 21:05, 7 August 2009 (UTC)
thread_title:  Graph of Wien's Law 

message: It would be nice to get an explanation of how this fits into the larger scheme of things: 
* Does this govern solids only?
* How does it relate to emission spectra of gasses?
* What about liquids, how do they glow? Plasmas?
* Objects in state transition or tri-points?
* Coloured, reflective, and translucent objects?
* How does it relate to an isolated object over time, as it's energy dissipates? How fast would something cool down?
I'm just layperson looking for a framework. -Craig Pemberton (talk) 19:30, 2 October 2009 (UTC)
thread_title:  Larger context 

message: Another layperson's request. The following sentence is intriguing: "Thermal radiation is generated when heat from the movement of charged particles within atoms is converted to electromagnetic radiation." 
#I am very interested in seeing the active voice version of this and think it would be a significant contribution to this article. 
#Isn't heat defined as the motion of particles? If so, then this explanation may be somewhat circular.
#The following doesn't explain it either and is probably incorrect but I hope it will get the ball rolling and give an idea of what I'd like to see here: "Thermal radiation is generated when charged particles within atoms collide as a result of heat energy and emit electromagnetic radiation." 
:Jojalozzo (talk) 13:51, 12 October 2009 (UTC)
replies: text: EM radiation is emitted whenever a charge undergoes some acceleration. Thermal radiation really comes from a wide variety of emission mechanisms - what they all have in common that makes them thermal radiation is that they're pumped by heat energy. Collision isn't really necessary for the process, and that doesn't really seem like a good way to explain it to me. Probably the best way to think about it is that jiggling a charge produces EM radiation and heat is jiggling. Tarchon (talk) 08:50, 17 February 2010 (UTC)
thread_title:  How is movement converted to light? 

message: From a person's perspective on the Earth there may seem to be definitive day and night times, but the world may also seem flat. Mention of the earth absorbing radiation from the sun during the 'day' and radiating it outward at 'night' should be replaced with a more accurate description of that movement of energy.
:Dkrocks (talk) 08:53, 7 December 2009 (UTC)
thread_title:  The Earth has no 'day' or 'night' 

message: It seems to me that this article is on exactly the same topic as incandescence. Should they be merged? If there's a distinction, what is it? --Steve (talk) 19:31, 16 December 2011 (UTC)
replies: text: For the time being, I edited incandescence to be a short article on the visible-light aspect of thermal radiation. I put lots of links so that readers looking for more detail will go to this article instead. --Steve (talk) 16:54, 19 December 2011 (UTC)

text: I see no reason why they should not be merged. --Cowlinator (talk) 02:08, 22 February 2017 (UTC)

text: I agree, having two seems redundant. KingSupernova (talk) 15:39, 17 May 2018 (UTC)
replies: text: Incandescence is the effect that warm objects emit thermal radiation, so incandescence is the effect / mechanism, while thermal radiation is the resulting electromagnetic fields / photons themselves. Regardless, I think it's the same topic so the articles should be merged.  — Preceding unsigned comment added by 92.67.227.181 (talk) 14:15, 4 July 2022 (UTC)
thread_title:  Merge with [[Incandescence]]? 

message: The fourth sentence in the first paragraph reads
This motion of charges produces electromagnetic radiation in the usual way.
That is the exact issue I wanted to know, and not being a SME I don't know the usual mechanism.  I presume in has to do with electrons being bounced between orbits (shells), but frankly don't know.  How would this work with molecules?  The page http://en.wikipedia.org/wiki/Electromagnetic_radiation on EM doesn't seem to say anything about "the usual way".  This sentence prevents non-SME's from comprehending the phenomena.  Please re-write or provide a link to "the usual way".  Thanx Bcwilmot (talk) 08:00, 11 June 2013 (UTC)
replies: text: I think the usual electrodynamical way was meant. According to the theory of electromagnetism, charged particles, such as electrons, induce an electric field which points outward, is strong near the particle and falls off in the distance. According to Maxwell's equations, the curl of the magnetic field is the change of the electric field, while the curl of the electric field in turn is the change of the magnetic field but in the opposite direction, so that the two fields induce one another. So an oscillating charged particle will induce an oscillating electromagnetic field: light. If you're interested in learning more about the subject, you could start by reading Introduction to Electrodynamics.  — Preceding unsigned comment added by 92.67.227.181 (talk) 14:13, 4 July 2022 (UTC)
thread_title:  The usual way....What? 

message: For example, fresh snow, which is highly reflective to visible light (reflectivity about 0.90), appears white due to reflecting sunlight with a peak wavelength of about 0.5 micrometers. Its emissivity, however, at a temperature of about -5 °C, peak wavelength of about 12 micrometers, is 0.99.
"reflecting sunlight with a peak wavelength of about 0.5 micrometers" -- Sunlight has this peak wavelength or the reflected light does? Or the reflectivity (a ratio) does?
"Its emissivity, however, at a temperature of about -5 °C" --  Temperature of the snow? 
"peak wavelength of about 12 micrometers" -- The peak wavelength of what? The emitted light? Or of the emissivity (a ratio)?  Sentence appears run-on.
178.38.186.135 (talk) 00:15, 9 April 2015 (UTC)
replies: text: 12 µm is long-wavelength thermal infrared radiation emitted by the snow because it's hundreds of kelvin above absolute zero.  — Preceding unsigned comment added by 92.67.227.181 (talk) 13:15, 4 July 2022 (UTC)
thread_title:  Ambiguous, confusing 

message: The article states "Whenever EM radiation is emitted and then absorbed, heat is transferred. This principle is used in microwave ovens, laser cutting, and RF hair removal."
Microwaves are waves of energy. When this energy hits an object molecules are put into motion creating heat locally. This is what the article on microwaves states.
Can someone explain this in more detail?
Artsiki  — Preceding unsigned comment added by Artsiki (talk • contribs) 18:04, 19 November 2016 (UTC)
thread_title:  Microwave Radiation 

message: I think this article would benefit from having a section connecting to the article on Photon statistics. It should include at least a brief discussion of super-Poissonian light and perhaps a discussion on the relation between the Bose-Einstein distribution and Planck's law (basically the latter is a special case of the B-E statistics, when one sets the energy 'quantum' to be E=\hbar \omega). Let me know if anybody is interested in collaborating! Mcasariego (talk) 14:46, 20 April 2020 (UTC)
thread_title: Section on quantum statistics?

message: This text blurb:
"The net radiative heat transfer from one surface to another is the radiation leaving the first surface for the other minus that arriving from the second surface."
violates 2LoT in the Clausius Statement sense. You're confusing some into thinking 2LoT can be violated nilly-willy, which a surprising proportion of them are using to claim that these continual 2LoT violations can cause catastrophic global warming. That's not scientific, does no one any good, and causes public perception of climate science to suffer.
2LoT (in the Clausius Statement sense... "No process is possible whose sole result is the transfer of heat from a cooler to a hotter body") states that energy cannot flow from a lower to a higher-energy region without external work being done upon the system... not via conduction, not via radiative means, not macroscopically, not at the quantum scale [1], not ever. Do keep in mind the definition of heat: "an energy flux". Thus: "No process is possible whose sole result is an energy flux from a cooler to a hotter body" without external energy doing work upon the system.
Radiation energy density is proportional to T^4 and is derived via the thermodynamic relation between radiation pressure p and internal energy density u, using the electromagnetic stress–energy tensor: p = u/3, it represents the EM field contribution to the stress–energy tensor, describes the flow of energy in spacetime, and is a representation of the law of conservation of energy.
At thermodynamic equilibrium, the Helmholtz Free Energy is zero, thus photon chemical potential is zero, thus no work can be done by the photon, thus no energy can flow:
F = U - TS
Where:
F = Helmholtz Free Energy
U = internal energy
T = absolute temperature
S = final entropy
TS = energy the object can receive from the environment
If U > TS, F > 0... energy must flow from object to environment.
If U = TS, F = 0... no energy can flow to or from the object.
If U < TS, F < 0... energy must flow from environment to object.
If U = TS, p_photon = u/3 = p_object, energy cannot flow because no work can be done. Photon chemical potential is zero.
"Therefore, at thermodynamic equilibrium, the chemical potential of photons is always and everywhere zero. The reason is, if the chemical potential somewhere was higher than zero, photons would spontaneously disappear from that area until the chemical potential went back to zero; likewise, if the chemical potential somewhere was less than zero, photons would spontaneously appear until the chemical potential went back to zero." [2]
Thus photons from a lower energy density source are not even incident upon a higher energy density object. [3]
In a cavity (remember that blackbody radiation used to be called cavity radiation) at thermodynamic equilibrium, quantized standing-wave wavemodes are set up. The wavemode nodes are always at the cavity walls. The walls and the standing-wave wavemodes in the cavity space have the same energy density, thus photon chemical potential is zero, thus the photons cannot be absorbed and re-emitted by the cavity walls, they are reflected. The same thing occurs out here in the real world. Energy does not and cannot flow from lower energy density to higher energy density without external energy doing work upon the system.
"Another point which, this time, lies quite within the domain where our formulae are significant refers to the scattering of light by an elastically bound electron. Consider again our assembly of harmonic oscillators of frequencies k which we may imagine to take the discrete but very finely distributed values determined by [ kL - η(k) = π N (N = 1, 2, 3... ) ]. Let all of them be in the ground state (energy 3/2 ħ k) with the exception of one (frequency k') which has the energy 5/2 ħ k', the vibration parallel to - say - the x-axis being excited by one quantum ħ k'. In this situation light of frequency k' coming from all directions is continuously scattered by the electron.
...the light quanta in the external field... which are affected by the presence of the electron through the appearance of the phase shift η. We might also call them phase shifted light quanta..." [4]
IOW, the electric field of a non-resonant photon, ~100,000,000 times smaller than the Coulomb field seen by the bound electron(s), slightly changes the phase of the bound electron(s), as equally as it changes the phase of the incident photon... no energy is exchanged if the photon is not absorbed, only the phase is shifted, changing the photon's vector. This is how reflection from a step potential works. This is dependent upon the differential between energy density of the photon and atom / molecule. In the case of absorption, the photon constructively interferes with the available resonant quantum states of the atom or molecule. No phase shift
"This means that, at every point of the region of space that is in (pointwise) radiative equilibrium, the total, for all frequencies of radiation, interconversion of energy between thermal radiation and energy content in matter is nil (zero)." [5]
Again, photons from a lower energy density source will not even incide upon a higher energy density object. Thus they cannot be absorbed. They are reflected, which increases ambient field radiation pressure until the objects and the ambient field have the same energy density, whereupon the objects can no longer emit nor absorb, they can only reflect the photons in the quantized standing wavemodes in the ambient field.
In fact, not all photons of sufficient energy to be absorbed are absorbed due to the angular momentum selection rules... all quantum numbers (including angular momentum) must be conserved in order for a quantum transition to occur.
It's not that the object is absorbing radiation from the cooler ambient (or cooler object) thus slowing its cooling rate, it's that the radiation pressure of the cooler ambient (or cooler object) lowers radiative flux from the warmer object, thus lowering its cooling rate.
[1] https://www.pnas.org/content/112/11/3275
[2] https://en.wikipedia.org/wiki/Chemical_potential#Sub-nuclear_particles
[3] https://objectivistindividualist.blogspot.com/2017/11/solving-parallel-plane-black-body.html
[4] http://www.solvayinstitutes.be/pdf/Proceedings_Physics/1948.pdf#page253
[5] https://en.wikipedia.org/wiki/Radiative_equilibrium
71.135.33.222 (talk) 01:07, 6 February 2021 (UTC)
thread_title:  Violation of 2LoT in the Clausius Statement sense 

message: https://en.wikipedia.org/wiki/Thermal_radiation
Thermal radiation is electromagnetic radiation generated by the thermal motion of particles in matter. Thermal radiation is generated when heat from the movement of charges in the material (electrons and protons in common forms of matter) is converted to electromagnetic radiation. All matter with a temperature greater than absolute zero emits thermal radiation. Particle motion results in charge-acceleration or dipole oscillation which produces electromagnetic radiation.
notice there is no source for this paragraph
another thing to note is that the charge particles are not accelerating from/in heat; kinetic energy is transferred without acceleration!
2600:1010:B126:C8FD:1D19:6704:7FB3:4EDB (talk) 09:57, 5 December 2021 (UTC)
replies: text: The current wording came to be in the edit on 21:44, 6 September 2021; we won't be able to tell you more without a link to your text. 92.67.227.181 (talk) 23:54, 7 July 2022 (UTC)
thread_title: did you plagiarize me?

message: The article should explain the apparent discrepancy between the colour table in the article and Color temperature. See the talk page of that article for details. 92.67.227.181 (talk) 13:08, 4 July 2022 (UTC)
thread_title:  Subjective color to the eye of a black body thermal radiator 

message: The article states right up front, "Thermal radiation is electromagnetic radiation generated by the thermal motion of particles in matter."
This seems to conflict with Timothy Boyer's statements: "Obvious­
ly a first step in creating a region of
vacuum is to eliminate all visible mat­ter, such as solids and liquids. Gases
must also be removed. When all mat­ter has been excluded, however, space
is not empty; it remains filled with elec­tromagnetic radiation. A part of the
radiation is thermal, and it can be re­moved by cooling, but another compo­nent of the radiation has a subtler ori­gin."
("The Classical Vacuum," Timothy Boyer, Scientific American, August 1985). 
So here we have a vacuum with no matter which still radiates thermal radiation. Care to discuss?  — Preceding unsigned comment added by Yates (talk • contribs) 01:30, 10 July 2023 (UTC)
replies: text: There is no contradiction really. Boyer is considering the inside of a cavity devoid of any matter. What he does not explicitly mention in this paragraph, is that the cavity is bounded by walls made of matter. Since photons do not interact with each other, the only way for the EM field to reach a thermal equilibrium is to interact with the walls of the cavity. The EM radiation inside the cavity is then exactly the radiation that is radiated by the walls of the cavity. Jähmefyysikko (talk) 05:20, 10 July 2023 (UTC)
thread_title:  Thermal Radiation