Riddle:
How is playing a game of darts like looking for planets around other stars?
To read the answer, hold a mirror to one side of your computer monitor so that you see the monitor's image. Or print this page and look at its reflection in a mirror.
Answer:
Catchy, huh? But, Dr. Marc, what does this strange answer mean?
And what do mirrors have to do with looking for other planets, Dr. Marc?
Almost all telescopes nowadays use mirrors, not lenses, to gather and focus the light from celestial objects. We have just seen, though, that mirrors reverse the image. The riddle's answer you saw on your monitor (or paper) looks very different from its image in a mirror. So why use mirrors?
Well, there are several excellent reasons. The optics (that is, the mirrors or lenses that focus light) of a good telescope must be nearly perfect. The mirror or lens must be just the right shape and the surface must be perfectly smooth and polished. In the case of a lens, even the inside must be perfect. Otherwise, the image appears distorted. Also, the larger the light-gathering surface (that is, the lens or mirror), the brighter the image. The brighter the image, the fainter the stars that can be seen.
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