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. In a dark room, the focal point can be clearly seen as the smoke from the paper scatters the light.
Engagement Suggestions
Invite students to predict what would happen if you used white paper rather than black.
- • Would it still burn?
- • Would it take more or less time to ignite?
Background
This demonstration illustrates two important points. It clearly shows that light can be focused to a point by a curved reflector. It is also an illustration of infrared radiation, and the connection between light and heat. When appropriate to the course, consider combining this with a discussion of the wavelengths of the electromagnetic spectrum, and the relationships of energy, heat, and temperature.