Light of intensity $10^{-5} \;wm^{-2} $ falls on a sodium photocell of surface area $2 cm^2$ Assuming that the top 5 layers of sodium absorb the incident energy, estimate the time required for photo electric emission in the wave picture of radiation . The work function of the metal is given to be about 2 ev . What is the implication of your answer ? Effective atomic area $=10^{-20} m^2$