Equipment:
Theory: A convex lens will refract parallel rays of light so that they converge to the focal point of the lens. A concave lens, on the other hand, will make the rays diverge away from the focal point.
Description: There are two lens available, one plano-convex and the other plano-concave. Mount the appropriate lens and the 5-ray box on the blackboard, powering the ray box with as much as 12 V DC.
Setup time: 5 minutes.