Three colored spotlights - red, green and blue - illuminate a white screen. A hand is placed between the screen and the lights, thus casting overlapping shadows on the screen. From left to right, you will observe a yellow, magneta and cyan shadow. The colors red and blue are produced where the shadows overlap. More info about this demonstration is available at The Physics Classroom: www.physicsclassroom.com/Class/light/u12l2e.cfm.