Blackbody as Bottle with Peephole?

A blackbody is a theoretical idealized object described as something "absorbing all incident radiation" commonly pictured as a cavity or empty bottle/box in which waves/photons are bouncing back and forth between walls at a certain temperature defining the temperature of the cavity. The bottle has a little peephole through which radiation is escaping to be observed, as indicated in the above common illustration of a blackbody.

A blackbody is supposed to capture an essential aspect of the radiation from a real body like the visible glow from a lump of iron at 1000 C, the Sun at 6000 C or the invisible infrared faint glow of a human body at 37 C.

But why is a lump of iron, the Sun or a human body thought of as an empty bottle with a peephole?

Yes, you are right: It is because Planck used this image in his proof of Planck's Law of blackbody radiation based on statistics of energy quanta/photons in a box. Planck's mathematical proof required a certain set up and that set up came to define the idealized concept of a blackbody as an empty bottle with peephole. But to actually construct anything near such a blackbody is impossible.

It is natural to ask if with another proof of Planck's Law, the concept of blackbody would be different, possibly closer to reality?

In Mathematical Physics of Blackbody Radiation I give a different proof of Planck's Law with a different concept of blackbody as a lattice of vibrating atoms absorbing and emitting radiation as electromagnetic waves, which models a real body like a lump of iron, and not a fictional empty bottle with a peephole.

This shows the role of mathematics in the formation of concepts of the World:

• With a strange mathematical proof the World may appear strange and incomprehensible.
• With a natural mathematical proof the World my become comprehensible.