Dne 04/12/2016 v 13:30 David Levy napsal(a):
'Poutnik[_5_ Wrote:
;1328995']
The closest approximation to the BB is a hole into a cavity.
Reflection or transmission of radiation coming into the cavity
through the smal enough hole is not measurable.


Please look at the following diagram:
http://tinyurl.com/lfnwltj
It is stated:
"An approximate realization of a black body as a tiny hole in an
insulated enclosure"

Why do we need this tiny hole/cavity?
Is it needed just to place the first radiation at that insulated
enclosure?
Let's assume that the insulated enclosure is isolated by 100%.
Let's also assume that we have the ability to close that tiny hole at
the moment that the first radiation gets in.
So, theoretically, the energy of the radiation will bump the walls
forever and at the same amplitude.
Now, if after some time we will measure that radiation.
Would it have a black body signature?

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.