In a photoelectric experiment a parallel beam of monochromatic light with power of $200W$ is incident on a perfectly absorbing cathode of work function $6.25ev.$ The frequency of light is just above the threshold frequency so that the photoelectrons are emitted with negligible kinetic energy. Assume that the photoelectron emission efficiency is $100\%$. A potential difference of $500V$ is applied between the cathode and the anode. All the emitted electrons are incident normally on the anode and are absorbed. The anode experiences a force $F = n \times {10^{ - 4}}N$ due to the impact of the electrons. The value of $n$ is $\underline{\hspace{2cm}}$. Mass of the electron ${M_e} = 9 \times {10^{ - 31}}\,kg$ and $1.0eV = 1.6 \times {10^{ - 19}}\,J.$