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rocapp

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## Homework Statement

A typical incandescent light bulb emits ~3x10^18 visible-light photons per second. Your eye, when it is fully dark adapted, can barely see the light from an incandescent light bulb 10 km away.

How many photons per second are incident at the image point on your retina? The diameter of a dark-adapted pupil is ~7 mm.

## Homework Equations

N = n*(d/D)

## The Attempt at a Solution

I found the above equation on the internet but it didn't work. I tried:

N = Number of photons*(Diameter/Distance)

=3x10^18 s^-1 * (7x10^-3 m / 1x10^4 m)

N= 2.1x10^12 photons/s

This is incorrect. Please help! Thanks!