4) The shortest wavelength of visible radiation is about 4000 A. What is the maximum kinetic with which an electron can escape from energy \text { (a) } \mathrm{Cs} \text { (work function } \Phi=1.9 \mathrm{eV} \text { ) } \text { (b) } \mathrm{Na} \text { (work function } \Phi=2.3 \mathrm{eV} \text { ) } \text { (c) } \mathrm{Al} \text { (work function } \Phi=4.1 \mathrm{eV} \text { ) } when it is irradiated with light of this wavelength?