The projector is used to enlarge an image onto a screen or wall. It contains a lamp and a concave mirror to illuminate the image and make it brighter. The lamp is placed at the centre of the mirror so that light rays reflected from the mirror travel back along their own path. To brighten the image further a condenser is used. The condenser consists of two half convex – also called plano convex lenses. The lenses focus the image onto the film. After passing through the film the light is focussed by a convex lens – the projection lens - onto a screen. The film is placed between the focal point and the centre of curvature so that the image is real
The image formed is upside down. In their infinite wisdom, scientists can correct for this deficiency in the laws of optics by turning the film upside down so that the image appears the right way up.