上海龙凤1314 shlf1996年8月托福阅读全真试题
Questions 1-10
上海龙凤1314 shlfThe 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
上海龙凤1314 shlfonto excess energy has been stimulated to emit it as light.
上海龙凤1314 shlfAlbert Einstein was the first to suggest the existence of
stimulated emission in a paper published in 1917. However ,
上海龙凤1314 shlffor many years physicists thought that atoms and molecules
上海龙凤1314 shlfalways were much more likely to emit light spontaneously and
上海龙凤1314 shlfthat stimulated emission thus always would be much weaker.
It was not until after the Second World War that physicists
上海龙凤1314 shlfbegan trying to make stimulated emission dominate. They
上海龙凤1314 shlfsought 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
上海龙凤1314 shlfColombia 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
上海龙凤1314 shlfEmission of Radiation. Although he thought of the key idea in
上海龙凤1314 shlf1951, the first maser was not completed until a couple of years
上海龙凤1314 shlflater. Before long, many other physicists were building masers
上海龙凤1314 shlfand 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
上海龙凤1314 shlfstimulated emission of visible light waves. At about the same time,
similar ideas crystallized in the mind of Gordon Gould, then a
37- year-old gradu