$4]$ The work function for potassium is $2\mathrm{.}25$ eV$.$ A beam with a wavelength of $400$ nm falls on a potassium layer and photoelectrons are emitted. $($a$)$ Find the maximum kinetic energy of the ejected photoelectrons, $[0\mathrm{.}85$eV$]($b$)$ Take the intensity of incident beam as $10-9$ W$/$m$2$ and find the number of electrons emitted per meter squared per second from the layer surface assuming that only $3\%$ of the incident photons are effective in ejecting electrons.$[6*107$el $/$s$*$m$2]$