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 wheels 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.

　　23. Paragraph 2 ___A___

　　24. Paragraph 3 ___B___

　　25. Paragraph 4 ___D___

　　26. Paragraph 5 ___C___

　　A. How is the nature of light explained today?

　　B. What are sources of light?

　　C. How did physicists measure the speed of light?

　　D. How does light travel?

　　E. How did people think of light years ago?

　　F. What causes a shadow?

　　27. Objects are visible to the human eye as light is __D____

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