Holograms are three-dimensional photographic images made with laser lights.
The idea of holograms was suggested by Hungarian-born British physicist Dennis Gabor in 1947. The idea could not be tried until laser light became available.
The first holograms were made by Emmett Leith and Juris Upatnieks in Michigan, USA, in 1963 and by Yuri Denisyuk in the Soviet Union.
Virtual reality headsets allow the viewer to see 3-D images by showing slightly different images to each eye. 118
Some holograms work in ordinary light, such as those used in credit cards to stop counterfeiting.
Holograms are used to detect defects in engines and airplanes, and forgeries in paintings by comparing two holograms made under slightly different conditions.
Huge amounts of digital data can be stored in holograms in a crystal.
In 1993 10,000 pages of data were stored in a lithium nobate crystal measuring just 1 cm across. 119
To make a hologram, the beam from a laser light is split in two. One part of the beam is reflected off the subject onto a photographic plate. The other, called the reference beam, shines directly onto the plate.
The interference between light waves in the reflected beam and light waves in the reference beam creates the hologram in complex microscopic stripes on the plate.
Some holograms only show up when laser light is shone through them.