1996年8月托福閱讀全真試題

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月托福閱讀全真試題】相關文章：

1997年5月托福閱讀全真試題04-23

（ＢＥＣ）高級閱讀全真試題05-04

（ＢＥＣ）中級閱讀全真試題05-04

劍橋商務英語BEC中級閱讀全真試題05-04

BEC中級全真試題05-04

托福模擬試題06-04

四年級英語閱讀訓練全真試題05-27

小升初數(shù)學全真模擬試題05-02

BEC高級寫作全真試題05-04

報關企業(yè)和全真試題及答案04-29