Black Body Model
Demonstrate that a cavity is blacker than any substance, and as such is a good approximation to an ideal black body.◙
Box with hole and two black spots.◙
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.
Martin Gardner, Physics Trick of the Month: Blacker than Black, TPT 31, 94 (1993).