# Homework Help: Photon Intensity

1. Sep 13, 2008

### jesuslovesu

1. The problem statement, all variables and given/known data
Supopse that a 100 W source radiates light of wavelength 600 nm in all directions and that the eye can detect this light if only 20 photons per second enter a dark adapted eye having a 7 mm diameter pupil. How far from the source can the light be detected under these conditions?

2. Relevant equations
$$E = hf = hc/\lambda$$

3. The attempt at a solution

I am having a hard time finding a relationship between the 100 W source and a distance. At what rate do the photons die off?

I can calculate the energy per photon: $$E = hc\ 600nm$$ but with that information, I still don't see a way factor distance in there.

Anyone have some hints?

how I can figure out at what rate it will dissipate over a distance.

2. Sep 13, 2008

### tiny-tim

Hi jesuslovesu!

Hint: imagine a sphere of radius r round the source.

How much energy falls each square metre of its surface?