posted by rick .
An owl has good night vision because its eyes can detect a light intensity as small as 5.0*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 7.0 mm and the light has a wavelength of 545 nm?
The energy in one second is what you are seeking.
Energy/second= 5.0E-13W/m^2 * area
Number photons= energy in a second/energy per photon.
The energy per photon= planck's constant*frequency= planck'sconstant*c/wavelength