Radiation – Do Pupils of an Eye Emit Blackbody Radiation?

Question

A blackbody, by definition is an ideal system that absorbs all radiation incident on it.

If a good approximation of a black body is a small hole leading to the inside of a hollow object, then am I right in saying that the pupils of an eye are a good approximation of a black body because they are also holes to a (near) spherical cavity?

If yes, do they also emit blackbody radiation in accordance with the Planck wavelength distribution function, and is this why they appear black?

Answer 1

Any approximation has a region of applicability - that is the conditions where one can apply it or not:

• Human eye obviously absorbs the radiation only in a certain range - e.g., it si totally transparent for gamma rays, which are also a part of the Planck spectrum (sinc ethe latter includes all frequencies up to infinity).
• Human eye reflects some of the radiation (due to the different refractive index of the lens, immediately behind the pupil (see this figure).
• Not all of the absorbed radiation is re-emitted (which is one of the conditions for the black body as related to the Planck's law).

Remark: Note also that one can define the black body radiation without resorting to a (largely historical) concept of the black body - as an equilibrium state of a photon gas: see, e.g., the discussion in this thread.