一级毛片免费不卡在线视频,国产日批视频免费在线观看,菠萝菠萝蜜在线视频免费视频,欧美日韩亚洲无线码在线观看,久久精品这里精品,国产成人综合手机在线播放,色噜噜狠狠狠综合曰曰曰,琪琪视频

TOEFL全真試題5-2

時間:2023-05-04 18:25:05 托福英語 我要投稿
  • 相關推薦

TOEFL全真試題(5-2)

Reading Comprehension

TOEFL全真試題(5-2)

Time: 55 minutes (including the reading of the directions). Now set your clock for 55 minutes.

Question 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

【TOEFL全真試題5-2】相關文章:

TOEFL試題06-22

BEC中級全真試題05-04

小升初數(shù)學全真模擬試題05-02

(BEC)高級閱讀全真試題05-04

BEC高級寫作全真試題05-04

(BEC)中級閱讀全真試題05-04

報關企業(yè)和全真試題及答案04-29

BEC口試全真試題(phase II)05-04

BEC口試全真試題(phase III)05-04

北京小升初數(shù)學全真模擬試題歸納05-02