A metal exposed to light of wavelength $800 \mathrm{~nm}$ and and emits photoelectrons with a certain kinetic energy. The maximum kinetic energy of photo-electron doubles when light of wavelength $500 \mathrm{~nm}$ is used. The workfunction of the metal is : (Take hc $=1230 \,\mathrm{eV}-\mathrm{nm}$ ).