- #1

Jenkz

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## Homework Statement

On a dark night, most people can see a 100W light bulb from at least 1 km away.

Given that a 100W light bulb emits about 5W of visible light, and assuming that

the wavelength is 500 nm, calculate the number of photons per second entering each

eye (pupil diameter 0.7 cm) of an observer 1 km from the bulb.

What is the average distance between photons en route from the bulb to the eye?

## The Attempt at a Solution

Energy of a photon= hc/lambda = 3.98 x 10 ^-19 J

Number of photons from 5w = 1.25x 10 ^19 per second.

area of eye = 3.85 x 10 ^-5 m^2

area of virtual sphere from bulb = 1.26 x 10 ^7m^2

Ratio of area of eye and virtual sphere = 3.06 x 10 ^-12

Total number of photons entering one eye = 3.85 x10^7 per second.

Now I'm not too sure what to do to get the average distance. Please help?