The minimum number of photons per second?

[girlygirl55]

Homework Statement

An owl has good night vision because its eyes can detect a light intensity as small as 4.5 10-13 W/m2. What is the minimum number of photons per second that an owl eye can detect if its pupil has a diameter of 7.5 mm and the light has a wavelength of 503 nm?

Homework Equations

I'm not really sure how to begin. Can you help me solve this step by step?

Photons per sec= Energy per sec/Energy per photon ??

The Attempt at a Solution

 

[diazona]

Photons per sec= Energy per sec/Energy per photon

Sure, that'd be a good start.