The word LASER is an acronym whose letters stand for " Light Amplification by Stimulated Emission of Radiation." Einstein's entirely theoretical prediction was made 43 years before the first laser was made. At that time when electronics was unheard of, transistors didn't exist and vacuum valves were still a novelty. Einstein predicted in 1917 that, under certain circumstances, an incident photon will generate another one , of exactly the same energy and hence the same frequency. Einstein added that in this type of emission both photons, old and new, will be in phase, will have the same polarisation and will propagate in the same direction.

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