Alight of wavelength $\lambda$  is incident on a metal sheet of work function  $\varphi =2eV$ .The wavelength  $\lambda$ varies with time as  $\lambda$ =3000+40t,  where  $\lambda$ is in $A°$  and t is in second.The power incident on metal sheet is constant at 100w. This signal is switched on and off for time intervals of 2 minutes and 1minute respectively . Each time the signal is switched on, the $\lambda$  start from initial value of $3000A°$  .The metal plate is grounded and electron clouding is negligible. The efficiency of photo emission is $1\%(hc=19840J-A°$  ).