I went to school in the early eighties and before dvd's and video machines and television sets came to schools, a slide projector was used to show us pictures. I remember that we had to go to the library classroom since it was the darkest room in school. The teacher would pull down this white screen , switch the projector on and turn off the lights. Sometimes he'd leave the classroom and then a whole lot of giggling would ensue from the back of the classroom. Still wonder what was going on back there

lol...demo shows how dark the room used to be

A slide projector is an opto-mechanical device to view photographic slides. Slide projectors were common in the 1950s to the 1970s as a form of entertainment; family members and friends would gather to view slide shows. In-home photographic slides and slide projectors have largely been replaced by low cost paper prints, digital cameras, DVD media, video display monitors, and video projectors.

A projector has four main elements:
electric incandescent light bulb or other light source (usually fan-cooled)
reflector and "condensing" lens to direct the light to the slide
slide holder
focusing lens

A flat piece of heat-absorbing glass is often placed in the light path between the condensing lens and the slide, to avoid damaging the latter. This glass transmits visible wavelengths but absorbs infrared. Light passes through the transparent slide and lens, and the resulting image is enlarged and projected onto a perpendicular flat screen so the audience can view its reflection. Alternatively, the image may be projected onto a translucent "rear projection" screen, often used for continuous automatic display for close viewing. This form of projection also avoids the audience interrupting the light stream by casting their shadows on the projection or by bumping into the projector.

It is also increasingly difficult in some countries to locate photo processors who will process slide film. Several manufacturers have stopped production of slide projectors.