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My question is simple if there is no perfect black body then what's the point of learning about it?

Because definition of balck body goes like this A body which can absorb and re emit all kinds of radiations

Then if a black body can absorb and emit all radiations where is the black body why cant a black body exist

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    $\begingroup$ You could conceivably ask this about all the simplified problems you learn of in physics, doesn't mean they aren't helpful. $\endgroup$
    – Triatticus
    Jan 15, 2022 at 11:44
  • $\begingroup$ BB is extremely important to chart it's spectrums when there is no net heat transfer aka thermal equilibrium but this is not easy to achieve since every material reflect a bit, please say you understand otherwise I'll blush super hard under the mask ;D $\endgroup$
    – user6760
    Jan 15, 2022 at 12:53
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    $\begingroup$ This is a valid question. I suspect it is getting down votes because it sounds disrespectful of physics. But to someone new to physics, it isn't obvious why it is useful. $\endgroup$
    – mmesser314
    Jan 15, 2022 at 14:59
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    $\begingroup$ There's also no frictionless surfaces, zero-resistant wires, pure quantum states, incompressible fluids, etc in real life. $\endgroup$
    – PeaBrane
    Jan 28, 2022 at 3:34

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To understand why learning about black bodies is useful despite their absence in nature it is useful to answer your second question first. The reason a black body can't exist it nature is because it's boundary, the outer layer, would have to be non-reflective and perfectly transmitting. This implies a few things but most importantly that the boundary needs to be infinity thin for the transmission and not smooth. It can't be smooth because radiation hitting a smooth boundary always needs to reflect some part of it's energy when transitioning to the new material. For some extra info on this see the Fresnel Equation.

So why do we learn about perfect black bodies? Well because of two reasons.

  • It is a model of reality that is easier to understand and does share some properties with reality. Or it's properties are related to the real properties via some parameter.
  • Near perfect black bodies, systems that are close enough to being perfect black bodies, do exist and studying them as if they are black bodies can teach us things about the system that would be much harder to discover without this idealisation.

In physics as a whole, and many other STEM fields, it's often not useful to look at details that won't change the overall picture. In relativity it's common to ignore the rest mass of hyper relativistic particles because it won't really influence the total energy of the particle because the total energy is much bigger then the rest energy. In thermodynamics the ideal gas law is used to model gasses despite an ideal gas not existing. However hot gasses are close enough to an ideal gas that as long as the conditions are right the ideal gas law will give correct predictions.

In short, yes perfect black bodies don't exist. But that does not make them a useless model of reality.

As this question's answers explain, one thing that can be modelled as a black body is a black hole. Which helps us study this phenomenon.

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  • $\begingroup$ Then can a black hole be defined as a black body because it absorbs everything $\endgroup$
    – Bharghavteja
    Jan 16, 2022 at 3:05
  • $\begingroup$ Yes they can, see this for more info on this topic. @Bharghavteja $\endgroup$
    – Dice
    Jan 17, 2022 at 10:33
  • $\begingroup$ I would like to give an obvious and explicit example of something really similar to a perfect black body: stars and planets. $\endgroup$
    – Lodinn
    Jan 17, 2022 at 11:21
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To give you a real-world example of where it is relevant: color temperature, going back to incandescent lamps. You probably see it while shopping for lightbulbs without realizing it, and the incandescent lamp radiation is fairly close to that of a black body. So, then, you can figure out what color would something be when you heat it up - rather helpful, is it not?

Another exciting aspect is that the black body approximation is the way to model energy balance for stars and planets, and it is extensively used in the search for habitable planets. Bit more exotic but I like to think it is important nonetheless.

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