The anode voltage of a photocell is kept fixed. The wavelength $\lambda$ of the light falling on the cathode is gradually changed. The plate current $\text{I}$ of the photocell varies as follows :

The minimum intensity of light that can be perceived by human eye is about ${10}^{-10} \mathrm{W} {\mathrm{m}}^{-2}$. The number of photons of wavelength $5.6\times {10}^{-7} \mathrm{m}$ that must enter the pupil of area ${10}^{-6} {\mathrm{m}}^2$ per second for vision is approximately (use $h=6.6\times {10}^{-34} \mathrm{J} \mathrm{s}$)