College Physics 10th Edition by Raymond Serway,Chris Vuille

Exercise 41

The threshold of dark-adapted (scotopic) vision is 4.0 × 10 11 W/m 2 at a central wavelength of 500 nm. If light with this intensity and wavelength enters the eye when the pupil is open to its maximum diameter of 8.5 mm, how many photons per second enter the eye