At 510 nm, the wavelength of maximum sensitivity of the human eye, the dark-adapted eye can sense a 100 ms long flash of light of total energy 4.0Af-10Acˆ'17J. (Weaker flashes of light may be detected, but not reliably.)
If 60% of the incident light is lost to reflection and absorption by tissues of the eye, how many photons reach the retina from this flash?