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  3. In A New Light: LEDs At the end of the 1800’s, Thomas Edison introduced the incandescent light bulb and changed the world. R

In A New Light: LEDs At the end of the 1800’s, Thomas Edison introduced the incandescent light bulb and changed the world. R

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问题                                         In A New Light: LEDs
    At the end of the 1800’s, Thomas Edison introduced the incandescent light bulb and changed the world. Remarkably, the incandescent bulb used today has changed little over a hundred years. An incandescent light consists of a glass bulb filled with an inert gas such as argon. Inside the bulb, electricity passes through a metal filament. Because of resistance, the filament becomes so hot that it glows. Given that 20％ of the world’s electricity is used to power lights, this represents an enormous amount of wasted energy.
    In the 1940’s, a new, more efficient form of lighting, the fluorescent bulb, was introduced. Fluorescents work by passing electrical current through gas in a tube, producing invisible ultraviolent light. A phosphor coating on the inside of the tube then converts the ultraviolent to visible light. Little heat is wasted. Fluorescents have proved popular in offices, factories, and stores, but they never took over the residential lighting market. The harsh color isn’t as pleasing as the warmer glow of incandescent lamps. Besides, they have a tendency to flicker on and off and to produce an annoying buzz.
    Now, lighting engineers are developing a new form of lighting that is both pleasing to the eyes and energy efficient. This is the light-emitting diode, or LED. LEDs are made up of layers of electron-charged substances. When an electrical current passes through layers, electrons jump from one layer to another and give off light without producing heat. Different types of materials result in light of different colors. Red, green, and orange LEDs have been used for decades in devices such as digital clocks, calculators, and electronic toys. In the future, however white-light-emitting diodes (WLEDs) may be used to light homes. Engineers say that they are significantly more efficient than either incandescent or fluorescent lights.
    Arrays of colored LEDs are beginning to be used in traffic lights and automotive lights. Today, colored light such as red brake light is created by shining a white incandescent light through a colored plastic filter. This is incredibly inefficient because only the red light that passes through the filter is used. The rest is wasted. Because LEDs actually produce red light, no filter is needed and no light is wasted. LEDs have other practical applications as well. For example, they can be used to light heat-sensitive materials like food or important documents.
    The next challenge for researchers is to develop an efficient, bright, inexpensive WLED. A few years ago, a Japanese scientist named Shuji Nakamura discovered that, by using layers of gallium nitride, he could create a powerful blue LED. Later, engineers devised two ways to use this blue LED to create a WLED. Red, green, and blue LEDs can be combined, creating a pleasant white light. Another way is to use a chemical coating similar to that inside a fluorescent bulb that converts the blue light to white. Nevertheless, it will still be some time before WLEDs are commonly used in homes. WLEDs are currently only twice as energy efficient as incandescent. They are also very expensive. But researchers believe that they can create WLEDs that are ten times as efficient and one thousand times as long-lasting as incandescent lights, making them cost effective.
    LEDs may someday have an even greater impact on developing countries than in the developed world. Worldwide, an estimated 2 billion people lack access to electricity. Lighting is usually provided by kerosene lamps. Kerosene is expensive, creates indoor pollution, does not provide very bright light, and worst of all, causes many fires. In India alone, 2. 5 million people were killed or injured annually in fires caused by overturned kerosene lamps. A low-energy (1-watt) WLED can provide enough light for a person to read by—more light, in fact, than most kerosene lamps. An entire rural village could be lighted with less energy than that used by a single conventional 100-watt light bulb. Energy to light these efficient LEDs can be provided by batteries that are charged by pedal-driven generators, or by solar energy. LEDs could revolutionize lighting to the same extent that the cell phone has revolutionized communication in places where land telephone lines are unavailable.
The word "they" in the passage refers to________.
选项 A、white-light-emitting diodes
B、digital clocks, calculators, toys, and similar devices
C、engineers
D、red, orange, and green LEDs
答案A
解析 事实细节题。they出现在第三段最后一句。其前一句讲到，未来，白光二极管有可能用于家庭照明。接着该句讲，工程师认为它们会比白炽灯或者日光灯效率提高很多。由此可知，这里的they指代白光二极管(white-light-emitting diodes)，故答案为A项。
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