BAAO — 2017 Astronomy & Astrophysics Challenge
Questions 9–10
Two light sources, A and B, emitting their light isotropically (i.e. equally in all directions) are placed at distance r and 2r respectively from a detector, which shows they have the same apparent brightness (i.e. bA/bB = 1). If A is moved to 2r and B is moved to 3r, what is the new ratio of apparent brightness b′A/b′B?
Light source A from the previous question is returned to a distance of r from the detector. How far away should it now be moved to appear 5 magnitudes fainter?
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Answer both parts to check. Both must be correct to count as correct.
Source
Source: British Astronomy and Astrophysics Olympiad (BAAO), Astronomy & Astrophysics Challenge 2017. Question text, answer-choice text, and answer were transcribed from the official BAAO paper and solutions PDFs; the explanation and metadata are original.