Time: 55 minutes (including the reading of the directions). Now set your clock for 55 minutes.
???The word laser was coined as an acronym for Light Amplification by the Stimulated
Emission of Radiation. Ordinary light, from the Sun or a light bulb, is emitted
spontaneously, when atoms or molecules get rid of excess energy by themselves, without
any outside intervention . Stimulated emission is different because it occurs when an
atom or molecule holding onto excess energy has been stimulated to emit it as light.
?? Albert Einstein was the first to suggest the existence of stimulated emission in a
paper published in 1917. However , for many years physicists thought that atoms and
molecules always were much more likely to emit light spontaneously and that stimulated
emission thus always would be much weaker. It was not until after the Second World
War that physicists began trying to make stimulated emission dominate. They sought
ways by which one atom or molecule could stimulate many other to emit light ,
amplifying it to much higher powers.
?? The first to succeed was Charles H.Townes, then at Colombia University in New
York . Instead of working with light , however, he worked with microwaves, which have
a much longer wavelength, and built a device he called a "maser" for Microwave
Amplification by the Stimulated Emission of Radiation. Although he thought of the
key idea in 1951, the first maser was not completed until a couple of years later. Before
long, many other physicists were building masers and trying to discover how to produce
stimulated emission at even shorter wavelength.
?? The key concepts emerged about 1957. Townes and Arthur Schawlow, then at Bell
Telephone Laboratories, wrote a long paper outlining the conditions needed to amplify
stimulated emission of visible light waves. At about