2017年托福考試全真試題練習

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2017年托福考試全真試題練習

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2017年托福考試全真試題練習

　　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 the same time, similar ideas

　　crystallized in the mind of gordon gould, then a 37- year-old graduate student at

　　columbia, who wrote them down in a series of notebooks. townes and schawlow

　　published their ideas in a scientific journal, physical review letter, but gould filed a

　　patent application. three decades later, people still argue about who deserves the credit

　　for the concept of the laser.

　　1. the word "coin" in line 1 could be replaced by

　　(a) created

　　(b) mentioned

　　(d) discovered

　　2. the word "intervention" in line 4 can best be replaced by

　　(a) need

　　(b) device

　　(d) source

　　3. the word "it" in line 5 refers to

　　(a) light bulb

　　(b) energy

　　(d) atom

　　4. which of the following statements best describes a laser?

　　(a) a device for stimulating atoms and molecules to emit light

　　(b) an atom in a high-energy state

　　(d) an instrument for measuring light waves

　　5. why was towne’s early work with stimulated emission done with microwaves?

　　(a) he was not concerned with light amplification

　　(b) it was easier to work with longer wavelengths.

　　(d) the laser had already been developed

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