I know that a black body is a hypothetical perfect absorber and radiator. It emits EM radiation with different intensities. But my doubt is, how can an empty container with a pinhole be considered a good assumption for a black body.

In Peter Atkins | Julio de Paula - Physical Chemistry, It's given that "A good approximation to a black body is a pinhole in an empty container maintained at a constant temperature: any radiation leaking out of the hole has been absorbed and re-emited inside so many times as it reflected around inside the container that is it has come to thermal equilibrium with the walls"

Does the hypothetical model allows accumulation of electromagnetic oscillators ? Isn't it's rate of absorption and emission be same?


1 Answer 1


The system intended to model the theoretical black body is not the cavity but the surface of a small hole on the cavity see for example wikipedia.

Of course, there are a few practical details which make a hole on a cavity more or less resembling the surface of a true black body.

First of all, the size of the hole must be large enough to reduce the fraction of the impinging radiation diffracted back (and consequently not absorbed). An opposite requirement is that the hole surface would be small enough to avoid the emission of a too large amount of energy as compared with the energy of the radiation inside the cavity.

In any case, the cavity should be large enough and walls characterized by good absorbing properties, to minimize the effect on the state of the internal radiation of the incoming beam and of the emission through the hole. In this respect, your concerns about a possible accumulation of energy in the electromagnetic modes or a mismatch in the emission/absorption rates, are key issues from the point of view of the practical realization of the best cavity in laboratory, but do not play any role on the conceptual side.

  • $\begingroup$ So, the small hole is the assumed blackbody and the container just has the fixed energy density? $\endgroup$ Commented Dec 26, 2018 at 11:48
  • $\begingroup$ Yes, the small hole is the real world approximation of the blackbody surface. The container plays different roles. It is the source of the outcoming radiation, and it is also the device to create radiation at thermal equilibrium. $\endgroup$ Commented Dec 26, 2018 at 11:54

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