Originally Posted by Poutnik[_5_]

The idea is easy, incoming radiation gets absorbed
by repeated non ideal absorption. That object behaving like this
actsas BB, followingn the thermodynamic laws.

As if not, there would not be possible the radiative equilibrium.

The is no sense ion closing it.
Neither the incoming rafiation would bounce forever,
but would be absorbed quickly.

The BB thermal radiation within the cavity is another topic.

--
Poutnik ( The Pilgrim, Der Wanderer )

A wise man guards words he says,
as they say about him more,
than he says about the subject.