PURPOSE: To show how an overhead projector works and to view its components.

DESCRIPTION: The Fresnel field lens can be removed, and the projector operated without it to illustrate its function. The vignetting effect without the field lens is very dramatic, as seen in the photographs below, taken with and without the Fresnel lens in place. The field lens focuses light passing through all points on the table through the adjustable lens and mirror system, so that the final image on the screen will be uniformly bright. Use a transparency grid on the table as the object.

SUGGESTIONS:

See Question of the Week #169 for information on using this demonstration to enhance class involvement.

REFERENCES: (PIRA unknown.)

EQUIPMENT: Prepared small Beseler overhead projector, transparency grid.

SETUP TIME: None.