Rotating disk and strobe demonstration.

Rotating disk and strobe demonstration. In a dark room, a stroboscope illuminates a rotating disk painted with three concentric sets of designs. The central design consists of circles ranging from small to large. The middle design is three slightly different rings of rectangles. The outer design is a series of dots within circles. The flashing strobe creates the illusion that the main disk is standing still, while the circles in the center ring alternately shrink and grow; the rings of rectangles rotate but at different speeds and directions, and the dots rotate either clockwise or counterclockwise within the outer circles. The strobe must flash at least sixteen times per second for the illusion to appear smooth to the eye. This effect, called 'persistence of vision', is the same mechanism that enables one to watch a motion picture movie composed of a series of individual pictures. Ref.:Modern College Physics by Harvey White, 6rd ed., p.26