托福英语 时间:2007-04-05 我要投稿
【 - 托福英语】

Reading Comprehension

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