The minimum intensity of light to be detected by human eye is 10−10Wm−2.  The number of photons of wavelength 5.6×10−7m entering the eye, with pupil area 10−6m2, per second for vision will be nearly

Intensity of light is given as I=PA=nEA, when n is number of photons entering the eye, E is energy of each photons entering the eye, E is energy of each photon and A is area of pupil. n=AIE=ΛIλhc, where λ is wavelength of photon.n=10−6×10−10×5×10−76.34×10−34×3×108=300Hence, minimum 300 photons need to enter the eye of a person to detect light.