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I am learning about black body radiation and it was stated in a textbook that

The best model for a black body is a large cavity with a very small hole in it which absorbs all incident radiation. The radiation hits the walls and rattles around inside and comes to thermal equilibrium with the walls before emerging from the hole.

However, the definition for a black body is an object that absorbs all incident radiation. In this case, for incoming radiation that did not hit the hole but the external walls of the cavity, they could either be reflected or absorbed. How then is the cavity a black body?

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    $\begingroup$ the inside is a black body, not the outside. $\endgroup$
    – JEB
    Dec 18, 2018 at 17:54
  • $\begingroup$ @JEB what do you mean? Inside as in the cavity or the inner walls? $\endgroup$
    – TaeNyFan
    Dec 18, 2018 at 17:56
  • $\begingroup$ idk, the hole is the black body, so when you ask about "radiation that did not hit the hole" what do you mean? $\endgroup$
    – JEB
    Dec 18, 2018 at 19:50

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I think in this case the author is referring to the small hole as being a good model for a black body. As mentioned any light which hits the hole will travel inside the cavity and reflect on the walls of the cavity until it is absorbed. The larger the cavity the more likely it is to absorb since it's less likely to be scattered back outside of the hole.

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