What is a hologram?

A hologram is a recording of light itself rather than a flat picture of an object. Ordinary photographs capture only the brightness and colour of light, but holograms also record the phase, the way light waves line up, using the interference pattern created when a split laser beam recombines. When the recorded pattern is later illuminated correctly, it reconstructs the original light field, producing an image that has genuine three-dimensional depth and shifts realistically as you move around it. The technique was invented by Dennis Gabor in 1948, work that later won him a Nobel Prize, though practical holograms only became possible after the laser was developed. Holograms are used today on banknotes and credit cards for security, in art, and increasingly in data storage and displays. Many so-called holograms on stage are actually clever optical illusions rather than true holograms.