A beam of light consists of four wavelength `4000Å , 4800 A, 6000 A` and `7000 Å`, each of intensity `1.5xx10^(-3)Wm^(-2)`. The beam falls normally on an area `10^(-4)m^(2)` of a clean metalic surface of work function `1.9 eV`. Assuming no loss of light energy (i.e. each capable photon emits one electron) calculate the number of photoelectrons liberated per second.