What it shows:
A black body box is an idealized black body radiator. That is, any light emerging from the interior of the box is purely a product of the box’s temperature.
How it works:
As the light emerging from the cavity should be a product of the temperature of the cavity, a cardboard box at room temperature should look black inside (emitting in the infra-red). To prove the point, Purcell’s box is painted white inside. A hole 6cm × 6cm is cut into the center of the lid, and to aid contrast, an area 25cm × 30cm surrounding the hole is covered in black felt.Looking in through the hole, the box appears black inside. Removing the lid will reveal the bright white interior.
Any box will do, provided it has a nice slip-on lid. Named after Prof. E. Purcell who’s idea it was, and what a great idea it is. Very useful illustration of the mysterious Black Body.