Photoelectric Effect: A Na vapor light emits 10W of light energy. Its wavelenght = 539nm and spreads in all directions. How many photons pass through your pupil (diameter 4mm) in 1s if you stand 10m away?

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anonymous · Answer

Okay, my initial approach was to find the total number of photons emitted per second by dividing the power (10 J/s) by the energy per photon (J/photon = hf = hc/lambda).
I then took this number and multiplied it by $$(.002^2*pi)/10^2$$ thinking that the area of the target is 4mm and the intensity of the light would drop off by r^2. This approach however was wrong according to my professor.

anonymous · Answer

He suggested I look up "solid angles" and work from there. My understanding of this advice is that the eye can be modeled as having a solid angle, theta, and the area the eye sees would be related to the following drawing|dw:1412347332925:dw|