The diagram shows the arrangement of lenses in a cinema or slide projector.
The light source is either a carbon electric arc or quartz iodine lamp which both give a small but high intensity light. The source is situated at the centre of curvature of a small concave mirror which reflects light that would otherwise be wasted on the projector housing. In order to obtain a brilliant picture on the screen, a condenser consisting of two plano – convex lenses collects light which would otherwise be spread out and be wasted, and causes it to converge through the slide on to the projection lens.
The projection lens is mounted in a sliding tube so that it may be moved to and fro to focus a sharp image on the screen.