You must have been to a cinema hall to watch a movie. Did you wonder how you saw moving pictures on the screen? A projector is a device used to show pictures on screen. One common type is device for showing photographic slides. The simplest projector consists of four major parts : a light source, a reflector that focuses the light, a focusing lens, and a projector lens.

A powerful light is required to show pictures on a screen. A projector uses an incandescent bulb that glows with heat. Some projectors use bulbs as strong as 1000 watts. The reflector, located behind the bulb, is concave (inward curving) mirror. It focuses the bright light rays through a thick plano-convex lens that is flat on one side and round on the other. The flat side of this lens faces the bulb. The light rays entering the focusing lens are bent inward and brought together. The rays then pass through a transparent photographic slide that is placed upside down between the focusing and projectio0n lenses. The projection lens turns the picture right side up and enlarges it.

Some kinds of projectors can be used with both slides and filmstrips. Some projectors can project drawings, maps, pages of books, and any other material that is opaque – that is, it is neither transparent nor on transparent film. These projectors are called opaque projectors.

NOTE: The first railway engine was built by the British engineer, Ricchard Trevithick, in 1804!