The naked eye can detect a stellar object of sixth magnitude in the night sky. With binoculars, we can see an object of the ninth magnitude. The sun’s brightness at Earth is 1400 W/m2. The Hubble Space Telescope can detect an object of the 30th magnitude, which amounts to a brightness of about 2 x 10-20 W/m2. (a) Consider a detector in the Hubble telescope with a collection area of 0.30 m2. If you assume hydrogen light of frequency 486 nm (blue-green), how many photons/s enter the telescope from a 30thmagnitude star? (b) An increase of magnitude one represents a decrease in brightness by a factor of 1001/5. Estimate how many photons/s from a sixthmagnitude star would enter your eye if the diameter of your pupil is 6.5 mm.

The naked eye can detect a stellar object of sixth magnitude in the night sky. With binoculars, we can see an object of the ninth magnitude. The sun’s brightness at Earth is 1400 W/m2. The Hubble Space Telescope can detect an object of the 30th magnitude, which amounts to a brightness of about 2 x 10-20 W/m2. (a) Consider a detector in the Hubble telescope with a collection area of 0.30 m2. If you assume hydrogen light of frequency 486 nm (blue-green), how many photons/s enter the telescope from a 30thmagnitude star? (b) An increase of magnitude one represents a decrease in brightness by a factor of 1001/5. Estimate how many photons/s from a sixthmagnitude star would enter your eye if the diameter of your pupil is 6.5 mm.

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