Holograms are photographic images that are three-dimensional and appear to have depth. Holograms work by creating an image composed of two superimposed 2-dimensional pictures of the same object seen from different reference points. Holography requires the use of light of a single exact wavelength, so lasers must be used. In reflection holograms, the kind of holography that can be viewed in normal light, two laser beams and a photographic plate are used to take an image of the object.
Both laser beams used in a holograph go through beam spreaders, which spread the laser light out like a flashlight. The coherence of the beam is lost, but it remains an exact wavelength. One beam illuminates the object from the side. The other beam, known as a reference beam, travels through a photographic plate and hits the object head-on, similar to the way in which a conventional camera takes a 2-D image. The reflecting light from the reference beam leaves an image, or hologram, on the photographic plate, but so does the light reflected by the object from the side beam. The result is a photographic plate that registers two images simultaneously, creating a hologram.
When viewing any object, the human eyes each receive a distinct image, from slightly offset reference points. The brain combines them into a three-dimensional image. The hologram produces the same effect artificially.
When developed, the hologram is printed onto a set of ultra-thin curved silver plates, which are made to diffract light. Diffraction is what a prism does -- it splits multi-wavelength white light into each specific wavelength. When white light hits the silver plates, the reflections diffract into each specific color, creating two offset images made up of colors that roughly reflect what the original object looked like. The result is a three-dimensional image, a hologram. The hologram is composed of the interference pattern between the two original images.
Because copying a hologram is extremely difficult, holograms are frequently used for security purposes on credit cards.