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In my physics class, I learned about "nonluminous objects" - these are objects which don't produce their own light. But, don't all objects emit light by black body radiation? So are all objects luminous objects? (except objects at 0K, if we get there somehow), or am I missing some point here?

Gert
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Not_CarlFriedrichGauss07
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    The concept of a "nonluminous object" is not rooted in any fundamental Physics. It's an instructional distinction that's useful for early science education but it isn't one that has any strong fundamental generality and you won't see Physicists making this distinction at all. This is a stepping-stone concept for children when learning about vision but it should be considered a throwaway concept that you replace with more accurate science as your understanding grows. By the time you know what the words "blackbody radiation" mean then luminous vs non-luminous becomes a deprecated concept. – J... Apr 20 '22 at 12:23
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    I kinda see that it is a poorly defined term as BowlOfRed points out, the fact that it's not rooted in fundamental physics makes sense. Thank you soo much !! @J... !!!!! – Not_CarlFriedrichGauss07 Apr 20 '22 at 13:54

3 Answers3

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Black bodies are in equilibrium with their surroundings - they absorb radiation from their surroundings and then re-emit it. Luminous bodies have internal energy sources, i.e., there is energy produced within these bodies, which is then emitted in the form of radiation. In this sense these bodies are not in thermal equilibrium, as there is a constant energy transfer from within the body to its surroundings, and this energy does not return back to the body.

Thermonuclear reactions within the Sun are an obvious example. Note however, that the radiation emitted in this way is reabsorbed/scattered many times before it reaches the surface, which results in the radiation spectrum being very similar to the black body spectrum - if the Sun is considered as isolated, the radiation reaching its surface can be considered as thermal/equilubrium radiation. However, the Sun is obviously not in thermal equilibrium with the surrounding vacuum (they have different temperatures - 6000K vs 0K).

See also How does radiation become black-body radiation?

Remarks:
@Quillo has pointed out in the comments that the energy is not necessarily generated within a luminous body, but could be simply stored in it (aka fossil heat) and being gradually released to the environment in a form of radiation. The body is still not in equilibrium with the surroundings, as it releases more energy than it absorbs.

@JonathanJeffrey has pointed out that the correct value for vacuum temperature is 3K, due to the microwave background radiation.

Roger V.
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    Thank you soo much @Roger Vadim !!!! , Thank you for your clear-to-point answer! , I am gonna look into this more, I have a lot to explore !!!! , Thank you again !! – Not_CarlFriedrichGauss07 Apr 19 '22 at 14:06
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    Very good answer. Just a minor point: is it really necessary to have energy production ? there may just be "fossil" heat. E.g. a red-hot piece of metal that has been heated and then just cools down (it is not in equilibrium with the surroundings). – Quillo Apr 20 '22 at 00:47
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    Minor point: the background temperature of space is actually around 3 K, due to the cosmic microwave background. – Jonathan Jeffrey Apr 20 '22 at 04:20
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    @Quillo thank you, I included your remark in my answer. – Roger V. Apr 20 '22 at 08:17
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    @JonathanJeffrey Thank you, I added this to the answer. – Roger V. Apr 20 '22 at 08:18
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    is a black-body inherently in equilibrium with its surroundings? – Tristan Apr 20 '22 at 10:19
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    @Tristan Black body radiation is radiation (photon gas) in thermal equilibrium. Black body is a historical device for deriving black body radiation in pre-quantum era. So, in principle, a black body is not defined as being in an equilibrium, but this usually means taking things out of context. You may also want to read this thread – Roger V. Apr 20 '22 at 17:12
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    @RogerVadim right, but the statement "black bodies are in equilibrium with their surroundings" seems misleading then, no? It's been a long time since I did any thermo though so it's entirely possible I'm just confusing myself – Tristan Apr 21 '22 at 09:39
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    @Tristan the statement is not precise, I will fix it. Thank you for pointing this out – Roger V. Apr 21 '22 at 12:10
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    So, this boils down to saying that a luminous object is one that is at a higher temperature than its surroundings? Also, I think you should write it as "3 K" rather than "3K"; the latter looks a bit like 3,000. – Acccumulation Apr 21 '22 at 23:46
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    @Acccumulation temperature is a limited concept, since it applies to objects that are in thermal equilibrium (at least approximately). Sun is not really in thermal equilibrium, but can be ascribed temperature, based on its spectrum. Neon lamp is more complicated. Also, luminous objects have to radiate - other types of energy exchange do kot make a body luminous. – Roger V. Apr 22 '22 at 05:24
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    What's interesting about this answer is that it does try to rationalize the idea of a "luminous object" by actually applying real physics. The definition is a good one, but it also leads to conclusions that would get you "wrong" answers on an elementary school test where this term might actually be seen. For example, by this definition a human being is a "luminous object", as would be any other endotherm. But a fifth grade teacher would tell a student that this is wrong and that a human is not a luminous object. – J... Apr 22 '22 at 13:26
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    @J... I didn't realize that it was about high school physics, but thought that it was about a similar concept in astronomy. Perhaps too advanced for a high school: human body is cooled via many non-radiative mechanisms, although it is certainly luminous in infrared range (I even have a couple of posts on this subject). This is why the concept is most meaningful when applied to stellar bodies, where the energy is lost almost exclusively via radiation. – Roger V. Apr 22 '22 at 16:04
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    @RogerVadim Sure, and it's a good answer in that context (+1 from me). I just think OP is coming from this from an earlier education standpoint - even pre-highschool where this is taught as part of introducing human vision, etc. See here for a paper, for example. – J... Apr 22 '22 at 16:33
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    @J... Yes... , It was a concept taught in 6th grade, in India, But for 10th (early bridge course) std, revision they taught us and didn't say anything about black body radiation edit:- 5th grade !!!!! , I will get it wrong if I say the human body is luminous in 10th grade too !!!!!!!!!!!!! – Not_CarlFriedrichGauss07 Apr 23 '22 at 05:32
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Yes, all objects emit radiation. But without other considerations, I would interpret "nonluminous" to simply mean that it isn't emitting visible light.

My cat is quite warm, but I would use "nonluminous" to describe it unless we were specifically discussing light as any form of EM radiation.

BowlOfRed
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As you say, all matter is radiating, even black holes! In principle, the phrase "nonluminous objects" should be qualified by what part of the spectrum they can be considered nonluminous and how much flux is needed to consider it "luminous". When people say "nonluminuous" they probably usually mean something like: the object emits less than 1 µW/m2 on the 400–700 nm range of wavelengths (in air or vacuum).

WaterMolecule
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    And it can even change depending on context. You typically wouldn't open your engine bay and say the motor is emitting light, but if you look at a cast iron exhaust manifold in the dead of night with all the lights turned off, it glows dimly. A performance car driven hard can glow brightly at night, but would still be hard to see during broad daylight. – MichaelS Apr 20 '22 at 21:50