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  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.


concave mirror

           light source               slide

                                              projector lens


  The small intense light source gets very hot and so to avoid damaging it you should never
  move a projector until it has cooled down. Most projectors have a cooling fan fitted which
  carries on running even after the light source has been turned off.