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Confusion With Blackbody Radiation

  1. Nov 21, 2015 #1
    A blackbody is a theoretical object that perfectly absorbs all the light that falls on it. From what I understand this is an ideal situation and does not actually exist in reality. Certain objects are close to being a blackbody but they do not absorb 100% of the light that hits it (i.e. some gets reflected).

    I have seen a few articles refer to a blackbody as a "perfect blackbody" or an "ideal blackbody". Isn't it redundant to describe a blackbody as being perfect or ideal? The defining property of a blackbody is that it completely absorbs all light that hits it.

    So this is how I understand it: a blackbody is a perfect absorber and all other object that aren't perfect absorbers of light are close-blackbodies or near-blackbodies. I found the following quote online and the terminology contradicts what I have read and understood from other sources. If anyone knows which one is actually correct can you please clarify it for me.

    "A perfect blackbody absorbs all wavelengths of incoming electromagnetic radiation perfectly; none of it is reflected. A perfect blackbody at a given temperature emits radiation in accordance with Planck's law. Any real blackbody is imperfect in both of these respects; it does not absorb all incoming radiation, and it does not have an emission spectrum that perfectly matches Planck's law."​

    A blackbody is also a good emitter of light. If this was not true, water placed next to the sun would freeze instead of evaporating away. Logically, this makes sense because the sun absorbs the energy around it including energy stored in the water. This isn't what we observe in real life so energy must be emitted from the sun back into its surroundings. I just don't understand why this has to be the case. Theoretically, why couldn't the sun just absorb the surrounding energy without emitting it back? In a similar fashion to black holes (this idea could be totally wrong, I don't really know much about black holes). Ultimately my questions is why do good absorbers have to be good emitters too?

    As a side note, would it be correct to call this system (the sun absorbing energy from its surroundings) endothermic?

    All objects above absolute zero emit radiation. The amount of radiation is dependent on the object's temperature. Do we call this radiation blackbody radiation? This is where I am really confused. I thought blackbody radiation was electromagnetic radiation emitted by a blackbody a theoretically perfect absorber and emitter.

    What would be some examples of objects found in nature that are very close to being a blackbody? So far I have stars/our sun and carbon black.
     
  2. jcsd
  3. Nov 21, 2015 #2

    mfb

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    Staff: Mentor

    It would violate the second law of thermodynamics.
    In a more microscopical way: a large emission means there are available energy transitions - those can be used to emit and absorb light in the same way. Or, alternatively: the laws of the universe are time-symmetric (at least to our current knowledge, and neglecting some details of particle physics not relevant here). A time-reversed emission is an absorption.

    Black holes do radiate (Hawking radiation) - they are extremely cold objects, but their spectrum should be the best blackbody spectrum we have in the universe. In practice we don't see it because the radiated power is incredibly tiny and everything around them is much hotter.
    If its spectrum is the same as the spectrum of a blackbody.

    For very hot objects, massive particles (like neutrinos, electrons and positrons) join the emission spectrum.
     
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