A projector is an instrument used top produce a large image of a small
object. The slide, or film, is placed behind the projector lens outside its focal length and is
illuminated but a small but powerful source of light.
The amount of light from the
source actually going through the transparent slide or film is increased by using a condenser
and a concave mirror.
The slide must be put into the projector upside down to give
an image on the screen which is the right way up.