When light waves are reflected from an object, the sizes (the amplitudes) of the waves, the frequency in which the waves occur (the wavelengths) and the phases of the waves: (the points where their peaks and troughs occur) are different, causing them to interfere with each other and thus affecting our visual perception of the object.
While a black and white photograph merely records the amplitude of light (as its intensity or brightness), and a colour photograph records both the amplitude and wavelength (in the form of brightness and colour), a hologram records both the phases and amplitudes of the light waves: in effect, the whole “light field” reflected by an object.
(Some types of hologram also record wavelength, resulting in coloured holograms, though they are often just monochromatic- usually green or red).
The result is an almost ghostly image of the “real object” in its original, solid, three-dimensional state, which usually appears to be floating in space “behind” the surface of the image.
Holograms have “parallax”- they appear different depending upon your viewpoint. The different view of a hologram that we see from each of our eyes creates the three dimensional effect. Therefore, the viewer can move back and forth in front of a hologram and see the image move as their viewpoint changes, as if they were looking at the original object itself.
Holography is not only used to record images, it can also be used to store information optically. In fact, two of the most commonplace data storage media, CD and DVD, have key aspects in common with holograms: they rely upon laser technology to function and incorporate a “diffraction grating”- a grid of microscopic ridges that interfere with the light that reflects from them. The differences in the light reflected from the surface of the discs allow for the data to be encoded. Another commonplace technology that owes much to holography can be found in the scanners used to read the prices of items from barcodes at supermarket checkouts ... read more ...