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  3. Learn about Light 1 Ancient civilizations were amazed by the existence of li...

Learn about Light 1 Ancient civilizations were amazed by the existence of li...

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问题 Learn about Light
1 Ancient civilizations were amazed by the existence of light for thousands of years. The Greek philosophers believed thatlight was made up of countless, tiny particles that enter the human eye and create what we call vision. However , Empedocles and a Dutch scientist named Christian Huygens believed that light was like a wave. According to them, light spread out and travelled like a straight line. This theory was accepted during the 19th century.
2 In 1905，Albert Einstein published a research paper in which he explained what is referred to as the photoelectric effect. This theory explains that particles make up light. The particles Einstein was referring to are weightless bundles (束）of electromagnetic (电磁） energy called photons (光子）_ Today, scientists agree that light has a dual ( 二重） nature — it is part particle and part wave. It is a form of energy that allows us to see things around us. .
3 Things that give off light are known as sources of light. During the day, the primary source of light is the sun. Other sources of light include stars, flames, flashlights, street lamps and glowing gases in glass tube.
4 When we draw the way light travels we always use straight lines. This is because normally light rays travel in a straight line. However, there are some instances that can change the path and even the nature of light. They are reflection, absorption, interference (干扰），etc.
5 Physicists have attempted to measure the speed of light since the early times. In 1849， Hippolyte Fizeau conducted an experiment by directing a beam of light to a mirror located kilometers away and placed a rotating cogwheel (旋转齿轮）between the beam and the mirror. From the rate of rotation of the wheel，number of wheel’s teeth and distance of the mirror, he was able to calculate that the speed of light is 313 million meters per second. In a vacuum (真空），however, the speed of light is 299，792，458 meters per second. This is about a million times faster than the speed of an airplane.
Paragraph 3 _________
选项 A How does light travel?
B How did people think of light years ago?
C How is the nature of light explained today?
D How did physicists measure the speed of light?
E What causes a shadow?
F What are sources of light?
答案F
解析本段中source—词重复出现，是本段阐述的重点。
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