Interference, diffraction, and holography

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Interference

Consider 2 waves meeting each other in the same space.  Their energies (amplitudes) can add or subtract.  This phenomenon is called interference.  If you've ever added sine waves on a calculator before, the effect is similar.

Crests can add to other crests, or cancel with troughs.  However, it is usually some combination (depending on the waves in question).  And often, beautiful "interference patterns" can result.

Diffraction is the phenomenon wherein light waves pass through small openings - the openings cause "new" waves to form, and these "new" waves interfere with each other.

Diffraction and Holography

Holography

Holography is a direct application of interference patterns - indeed, it is the recording of an interference pattern on film, reconstructed with a laser.

Holography is an interference phenomenon caused by two beams - a reference beam (coming from a laser), and an object beam (which reflects off the object).  This interference pattern is burned into the film emulsion of the holographic film.  It can be reconstructed when light passes through it again.