Light is incident normally on a completely absorbing surface with an energy flux of $25\mathrm{ }\mathrm{W}\mathrm{ }\mathrm{c}{\mathrm{m}}^{-2}$ . If the surface has an area of $25\mathrm{ }\mathrm{c}{\mathrm{m}}^2$ , the momentum transferred to the surface in $\text{40}\text{min}$ time duration will be:

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 $h=6.6\times {10}^{-34} \mathrm{J} \mathrm{s}$)