Blackbody Radiation and Emissivity Explained

[Wannabeagenius]

Hi All,

I'm reading about blackbody radiation and emissivity and I came upon the following statement

"Imagine looking into a small opening of a deep cave. In the visible wavelengths, the opening looks black because the light that enters the cave is not easily reflected back out. However, the cave glows with emitted thermal IR energy. This energy emerges as a complete spectrum of all wavelengths of IR light. The radiance at each infrared wavelength is the maximum amount possible for a given temperature."

Why does the IR energy that emerges contain the complete spectrum of all wavelengths of IR light and why is the intensity at each wavelength the maximum possible? Why does it behave like a perfect blackbody?

Thanks,
Bob

 

[NobodySpecial]

Why does the IR energy that emerges contain the complete spectrum of all wavelengths of IR light

Classically it actually contains all wavelengths of all light - it just peaks in the IR

and why is the intensity at each wavelength the maximum possible?

Slightly odd wording - the intensity at each wavelength if it's a blackbody is exactly what the blackbody curve predicts it should be

Why does it behave like a perfect blackbody?

If no outside light is reflected back out without being absorbed by the walls and any light you do see has been emitted by the walls then you get a pretty good blackbody.

 

[granpa]

light entering the cavity bounced back and forth till its almost completely absorbed

emission is just the reverse of absorption