The work function of a metal is 1.6×10−19J. When the metal surface is illuminated by the light of wavelength 6400∘A , then the maximum kinetic energy of emitted photo-electrons will be (Planck's constant h=6.4×10−34 Js)
1.4×10−19J
Kmax=hcλ−W0=6.4×10−34×3×1086400×10−10−1.6×10−19
=1.4×10−19J