Slide projectors work by shining light through specialized photographs called slides. When the light goes through the slide, it enlarges the photograph and displays the image on a screen or on the wall. Usually, slide projectors use a carousel or magazine to hold many different slides. When a button is pushed, the slides advance in consecutive fashion.
Inside the body of the slide projector, there are several lenses and mirrors. The slide projector relies on these lenses and mirrors to project roughly parallel rays of light through the slide. On the other side of the slide, a lens spreads the light rays back out, which enlarges the image. Typically, slide projectors feature an adjustable lens so that the operator can bring the image into sharp focus. The farther away the slide projector is from the screen or blank wall, the larger the image will appear to be. Conversely, if the slide projector is placed close to the screen, the image will not be large.
Modern slide projectors are digital devices. Instead of shining light through a slide, the image comes from a computer that is hooked up to a projector. In contrast to analog slide projectors, digital projectors are controlled via the computer.