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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