An overhead projector (OHP) is a device that uses light to project an enlarged image onto a screen, allowing a large audience to view a small document or picture. Here’s how it works:

1. Image Source: The OHP uses a page-sized sheet of transparent plastic film (also known as “viewfoils,” “foils,” or “transparencies”) with the image to be projected. This transparency can either be printed or hand-written/drawn.

2. Optical System: The OHP operates on the same principle as a slide projector. A focusing lens projects light from an illuminated transparency onto a projection screen, forming a real image. However, there are some differences due to the larger size of the transparencies used (generally the size of a printed page). The transparency is placed face up (toward the mirror and focusing lens) so that it’s readable to the presenter.

3. Mirror and Lens: To accommodate the larger transparency size, the OHP includes a mirror just before or after the focusing lens. This mirror folds the optical system toward the horizontal. It also reverses the image so that what’s projected onto the screen corresponds to what the presenter sees when looking down at the transparency. Unlike 35mm slide projectors, where the slide’s image is non-reversed, the OHP ensures the correct orientation for the audience.

4. Condenser: Since the focusing lens is much smaller than the transparency, an optical condenser plays a crucial role in illuminating the transparency. The condenser redirects most of the light hitting it into a converging cone toward the focusing lens. A Fresnel lens is commonly used for this purpose. Without the condenser, most of the light would miss the focusing lens.