The Human Eye Can Just Detect Green Light of Wavelength $500 \mathrm {~nm}$

Short Answer

The human eye can just detect green light of wavelength $500 \mathrm {~nm}$ if it arrives at the retina at the rate of $2 \times 10 ^ { - 18 } \mathrm {~W}$ . How many photons arrive each second? $\left( c = 3.0 \times 10 ^ { 8 } \mathrm {~m} / \mathrm { s } , h = \right.$ $\left. 6.626 \times 10 ^ { - 34 } \mathrm {~J} \cdot \mathrm { s } \right)$