Viewing the Moon

Electricity and Magnetism Level 3

The average intensity of the full moon is 2.92 × 10 − 4 W / m 2 2.92 \times {10}^{-4} W/{m}^{2} . Approximately how many photons enter the eye per second whenever we look at the full moon on a clear night?

Assumptions

1) The area of the pupil is 3 × 10 − 5 m 2 3 \times {10}^{-5} {m}^{2}

2) The wavelength of the moonlight is 550 nanometers

5.26E10 2.38E10 1.57E10 7.22E10

Steven Zheng by Steven Zheng , 26, China

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3 solutions

This is easy as hell and can be done with simple dimensional analysis. Why is this level 5?

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Don't ask me, I initially set it to level 3. I guess people guessed wrong?

Steven Zheng - 6 years, 5 months ago
Jake Lai
Dec 11, 2014

Pretty easy to find the power and hence energy of the full moon in a second.

Use E γ = h c / λ E_{\gamma} = hc / \lambda to find the energy of a photon and find E m o o n / E γ E_{moon} / E_{\gamma} .

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Michael Mendrin
Oct 25, 2014

You know, not only there isn't an exact universally agreed "peak sunlight wavelength" (it can vary from 500 nm. to 530 nm from different references), the diameter of the human pupil can vary considerably from about 1.5 mm to 8 mm, from bright to dim light conditions. Those things should have been specified in stating this problem.

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