dis8-slides

dis8-slides - ASTR 100 Discussion 8 Oct. 31, 2011 1....

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ASTR 100 Discussion 8 Oct. 31, 2011
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1. Imagine that concentric spheres are placed around the Sun at radii of 1, 2, and 3 units. (This is shown in the diagram below.) If the apparent brightness of the Sun at the patch on shell 1 is 16, then what is the apparent brightness of the Sun at the patch on shell 2 marked with an X? Give a numerical answer, but EXPLAIN YOUR REASONING IN WORDS.
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Answer: This excellent answer was written by a student in the class: “The Inverse-Square Law gives the equation relating luminosity, apparent brightness, & distance. b = L/(4 d 2 ). Using this equation we see is a constant. So when d changes from 1 to 2, d 2 goes from 1 to 4, meaning the apparent brightness will decrease by a factor of 4 and become 4 from 16.”
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Grading checklist: Did you give a numerical answer? Did you discuss how distance affects brightness? Did you remember to discuss the fact that distance squared is important?
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2. Imagine that concentric spheres are placed around the Sun at distances of 1, 2, and 3 units. If the apparent brightness of sunlight at the first patch (on shell 1) is 18, then what is the apparent brightness through the patch marked with an X on shell 3? Give a numerical answer, but explain your reasoning in words.
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Answer: This excellent answer was written by a student: “Using the Inverse-Square Law, we get that apparent brightness is inversely proportional to the distance squared. So if at distance 1, the sunlight has [an] apparent brightness of 18, then at shell 3, the apparent brightness decreases by a factor of 3 2 = 9, so on the patch marked X on shell 3, the apparent brightness is 18/9 = 2.”
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Grading checklist: Did you give a numerical answer? Did you discuss how distance affects brightness? Did you remember to discuss the fact that distance squared is important?
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By observing the light emitted by a star, we can infer its composition from A. the lines in the star's spectrum. B.
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dis8-slides - ASTR 100 Discussion 8 Oct. 31, 2011 1....

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