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chap-03 - CH 03 pp64-94 12:02 PM Page 64 WE B LINK 3.1 CH...

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W E B L I N K 3 . 1 CH 03 pp64-94 6/5/02 12:02 PM Page 64
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65 Chapter 3 Light and Telescopes WHAT DO YOU THINK? 1 What is light? 2 Which type of electromagnetic radiation is most dangerous to life? 3 What is the main purpose of a telescope? 4 Why do stars twinkle? 5 What type(s) of electromagnetic radiation can telescopes currently detect? he telescope is the single most important tool of astronomy. Using a telescope, we see objects in space far more brightly and clearly and at greater distances than we can with the naked eye. Telescopes have played a major role in revealing the universe since Galileo viewed the craters on the Moon four centuries ago. Refracting telescopes, which use large lenses to collect incoming starlight, were popular with astronomers 150 years ago. Modern astronomers strongly prefer reflecting tele- scopes, which gather light with large curved mirrors. In either case, the main purpose of a telescope is to collect as much light as possible. Astronomers attach a variety of equipment to telescopes to record incoming starlight. THE NATURE OF LIGHT Visible light is the form of electromagnetic energy to which our eyes are sensitive. But what exactly is visible light? How is it produced? What is it made of? How does it move through space? Scientists have struggled with these ques- tions for the past 400 years. 3-1 Newton discovered that white light is not a fundamental color and debated whether light is particles or waves From the time of Aristotle until the late seventeenth century, most people believed that white was a fundamental color of light. The colors of the rainbow (or, equivalently, the colors created by light passing through a prism) were believed to be added somehow as white light went from one medium through another. Isaac Newton performed experiments dur- ing the late 1600s that provided our first insights into the nature of light and disproved these earlier beliefs. Newton started by passing a beam of sunlight through a glass prism, which spread the light out into the colors of the rainbow, as shown in Figure 3-1a. This rainbow, called a spectrum (plural; spectra ), sug- gested to Newton that white light is actually a mixture of all the primary colors and that these colors separate out by changing direction or refracting by different amounts while going through the prism. He then selected a single color and sent it through a second prism (Figure 3-1b). The light emerging from this prism was the same color, heading in a different direction. After discovering that individual colors could not be further separated, Newton recombined all the colors, thereby re-creating white light. So different colors are different entities. But what? The two major issues were determining whether they were parti- cles or waves and how rapidly light traveled.
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