Purpose:Illustrate focusing of heat in a very dramatic way.
Description:This demonstration uses one of the old overhead transparency projectors that focuses the light by a large parabolic mirror under the platform (rather than a Fresnel lens on the platform as in newer models), as seen in the images above. The heat filter and the mirror system above the projector have both been removed. There is sufficient heat focused about two feet above the projector to burn a piece of black paper in a few seconds.
Invite students to predict what would happen if you used white paper rather than black. would it take more or less time to ignite? Would it burn at all?