energy per photon (一個光子能量)\(E=\frac{hc}{\lambda}=\frac{(6.63*10^{-34}J*s)(3*10^8m/s)}{550*10^{-9}m}=3.62*10^{-19}J\)

100 photons per second

Threshold power for the eye \(P=E*100/s=3.62*10^{-17}W\)