Equipment:
Theory: Parallel rays of light striking a convex mirror will be reflected so that they diverge; parallel rays reflecting from a concave mirror will converge to the focal point.
Description: Mount the curved mirror, which is concave on one side and convex on the other, and the 5-ray box on the blackboard, powering the ray box with as much as 12 V DC. Using the concave mirror, it is easy to see that the reflected beams converge at a point. With the convex mirror, you can extend the reflected beams back behind the mirror with chalk to show that they meet at the focal point.
Setup time: 5 minutes.