PURPOSE: Demonstrate that a cavity is blacker than any substance, and as such is a good approximation to an ideal black body.
DESCRIPTION: A hole cut in a box which is painted black inside is flanked by two similar size patches of black paper and black felt. The hole appears darker than the two black surfaces, even when the surfaces are clean.
SUGGESTIONS: See Question of the Week #92 for information on using this demonstration to enhance class involvement.
REFERENCES: (PIRA unknown.)
EQUIPMENT: Box with hole and two black spots.
SETUP TIME: 10 minutes.