THE SCIENCE BIT

Holograms are made using a coherent light source, as opposed to a white light source, which ultimately means a laser beam.  (For an explanation of coherent light sources see explanations)

If a white light source were used to create the hologram, i.e. a light bulb, then the image produced lacks the intensity and depth of image associated with the modern hologram (see History).  For this reason a Laser is used.

Taken from http://www.holo.com/holo/book/book1.html#bascon

The beam from the Laser is split into two, a reference and an object beam.  The object beam is distorted by the shape of the object that is being recorded.  The reference beam remains undisturbed.  What is recorded on the film is the interference pattern between the two beams. (For an explanation of interference see explanations)

The plate to which the hologram is recorded is usually made of AgI.  When the beams interfere constructively then the light intensity is increased and the AgI molecules experience greater photoexcitation.  This pattern of interference imparts a molecular grating on the plate.  If the plate is then viewed using light from the same direction as the reference beam  then the image of the object appears on the plate.