- 相关推荐
1996年8月托福阅读全真试题
Questions 1-10
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 the same time,
similar ideas crystallized in the mind of Gordon Gould, then a
37- year-old gradu
【1996年8月托福阅读全真试题】相关文章：
1997年5月托福阅读全真试题04-23
（ＢＥＣ）高级阅读全真试题05-04
（ＢＥＣ）中级阅读全真试题05-04
剑桥商务英语BEC中级阅读全真试题05-04
BEC中级全真试题05-04
托福模拟试题06-04
四年级英语阅读训练全真试题05-27
小升初数学全真模拟试题05-02
BEC高级写作全真试题05-04
报关企业和全真试题及答案04-29