2. 外语
  3. 　　 Most sources of illumination generate light over an appreciable period, and indeed if 　　 an object is lit for a very brie

　　 Most sources of illumination generate light over an appreciable period, and indeed if 　　 an object is lit for a very brie

admin2005-01-30  78
问题 　　     Most sources of illumination generate light over an appreciable period, and indeed if
　　 an object is lit for a very brief time(less that 1/25 second), the human eye will not react
　　 in time to see the object. A photographic emulsion---that is, a light-sensitive coating on
　　 photographic film, paper, or glass--will, however, record much shorter bursts of light. A
(5) photographic flash can therefore be used to capture high-speed movement on film as well
　　 as to correct deficiencies of the normal surrounding lighting. Photoflash is now generated
　　 electronically, but the earliest form, first used in 1864, was a paper bag containing
　　 magnesium wire and some oxygen-rich substance, such as potassium chlorate. When the
　　 bag was ignited, the metal burned with an intense flash. A contemporary observer reported
(10) that "this quite unsafe device seems to have done nothing worse that engulf the room in
　　 dense smoke and lead to pictures of dubious quality and odd poses."
　　     The evolution of the photoflash was slow, flashbulbs, containing fine wire made of a
　　 metal, such as magnesium or aluminum, capable of being ignited in an atmosphere of pure oxygen
　　 at low pressure, were introduced only in the 1920’s.In the earliest type, the metal
(15) was separated from the oxygen by a thin glass bulb. The dash was fired by piercing the
　　 bulb and allowing the oxygen to come into contact with the metal, which ignited
　　 spontaneously. Later bulbs were fired by an electric battery, which heated the wire by
　　 passing a small current through it. Other combinations, such as the pairing of oxygen
　　 difluoride with zirconium, have also been used. In each case enough energy is given out to
(20) heat the oxidizable metal momentarily to a white-hot emission of visible light. The smoke
　　 particles are so small that they cool rapidly; but since they are white, they contribute to the
　　 brilliance by reflecting the light from their still-glowing neighbors. A slightly bigger form
　　 of the metal will burn for a longer time.
选项 A、The history of the photoflash
B、Theories about how the eye reacts to light
C、The technology of modern photography
D、The dangers of using the early photoflash
答案A
解析
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