How far from the lightbulb can you stand to see it? photon problem

[okgo]

Homework Statement

http://www.screencast.com/users/trinhn812/folders/Jing/media/23298f1b-2ef4-4cdf-8b7a-58c9be07335a

Homework Equations

E=hf

The Attempt at a Solution

I'm not even sure how to get the distance for this problem.

 

[Delphi51]

Looks like an interesting problem, okgo.
And you have a good start.
Those photons are spread over the surface area of a sphere at distance R from the light bulb. You should be able to write an expression for the number of photons per square meter at radius R. Equate that to the specified 20 per (area of pupil).
Then you can solve for R.