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Which of the following is an advantage to using a concave mirror in the construction of a telescope?
  1. The telescope can gather more light than a telescope using lenses.
  2. The telescope does not suffer from chromatic aberration.
  3. The telescope can provide greater magnification than a telescope using lenses.
  1. I and III only
  2. II only
  3. I and II only
  4. I, II, and III
Question by OpenStax is licensed under CC BY 4.0.
Solution Video

OpenStax College Physics for AP® Courses Solution, Chapter 26, Problem 9 (Test Prep for AP® Courses) (2:31)

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Video Transcript
This is College Physics Answers with Shaun Dychko. We want to know what are the advantages of using a concave mirror in the construction of a telescope instead of using glass lenses. Well, number one, the telescope can gather more light because it's possible to make mirrors much much bigger than it is possible to make lenses. A lens would become really heavy if it was big and it will be difficult to machine it perfectly. Whereas, a mirror can be more easily constructed on a large scale. So, the number one is true. All three are true here, but let's explain why. Number two, the telescope does not suffer from chromatic aberration when using a mirror. Chromatic aberration refers to the fact that there is a certain dispersion in a glass lens. So, dispersion means that different colors get refracted by different amounts. So, the index of refraction of this glass is different for different colors. So, the green light, say, might get refracted here. Whereas, a blue light might experience a greater index of refraction and get focused closer to the lens than the green did. Red, likewise, will also be different and that it would be refracted less than the green. And so, lenses have different indexes of refraction and therefore different focal lengths for different colors and this difference in focal lengths of different colors is called chromatic aberration. Of course, if you're looking at an object that had only one color, then it wouldn't matter. But most things have multiple colors, and so a mirror has the same angle of reflection for all colors and it doesn't matter what the color is. The focal length of the mirror will be the same. And so, that's an advantage that a mirror has versus a glass lens. And then number three, the telescope can provide greater magnification than a telescope using lenses because the focal length for a mirror can be made very large because it's possible to construct a mirror that's only very slightly curved and so, with only a very slight curve, the focal lengths can be really far away. Whereas, with a glass lens, it's much more difficult to do. So, one, two, and three are all true and there we go.