Physics behind Eyes and Light

A light bulb rated at \(100W\) is \(20\)% efficient. (i.e. \(80\)% of its output is invisible infra-red radiation and only \(20\)% is visible light.) A person can see the light with his naked eye from a distance of \(10km\) on a dark night. If given that the area of the pupil of the person's eye is \(0.50cm^2\), what is the power of the visible light received by the eye of the person?

Details:

  1. Give your answer in Watts/\(W\).
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