Revolving window illusion.

Revolving window illusion. The revolving trapezoidal window is a striking optical illusion in perspective. The window is plywood painted white, black and gray, with rectangles cut out for windows. It is mounted on a slowly rotating motor-driven shaft in a darkened room. (There are light sources from the front and from the side, shielded from the class.) The window appears to rotate first in one direction, and then the other. An additional effect is to hang a brightly colored ball on the upper corner of the small end. The ball appears to keep going in the same direction while the window oscillates back and forth. One other effect is to place a colored rod through the upper center window. As the assembly rotates, the rod's behavior is baffling. Ref.:Modern College Physics by Harvey White, 6rd ed., p. 24-25