Overhead Projectors

An overhead projector is a variant of slide projector that is used to display images to an audience.
An overhead projector typically consists of a large box containing a very bright lamp and a fan to cool it. On top of the box is a large fresnel lens that collimates the light. Above the box, typically on a long arm, is a mirror and lens that focusses and redirects the light forward instead of up.
Transparencies are placed on top of the lens for display. The light from the lamp travels through the transparency and into the mirror where it is shone forward onto a screen for display.
The mirror allows both the presenter and the audience to see the image at the same time, the presenter looking down at the transparency as if writing, the audience looking forward at the screen.
The height of the mirror can be adjusted, to both focus the image and to make the image larger or smaller depending on how close the projector is to the screen.