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This wikipedia article on the "cavity with a hole" model of a black body says:

The hole is not quite a perfect black surface — in particular, if the wavelength of the incident radiation is longer than the diameter of the hole, part will be reflected. Similarly, even in perfect thermal equilibrium, the radiation inside a finite-sized cavity will not have an ideal Planck spectrum for wavelengths comparable to or larger than the size of the cavity.

So I have 2 questions.:

Why is it that light of a wavelength larger than the hole will be reflected?

Why are wavelengths larger than the size of the cavity not emitted?

FizzleDizzle
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1 Answers1

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The propagation of an EM wave through a small hole decays very quickly, like $d^4$, so while it can propagate it's very unlikely. Same principle that makes the microwave mesh a good way for us not to be microwaved too.

Related question: Why are electromagnetic waves not able to pass through a hole with a diameter smaller than the wavelength?

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