The maximum kinetic energy (Kmax) of photo-electrons emitted in a photoelectric cell is measured using lights of various frequencies n. The slope of the graph is equal to

charge on an electron                                                 work-function of emitter velocity of photon  Planck’s constant

Planck’s constant

hf = W0 + (K.E.)             or   K.E. = Kmax = hf – W0             Comparing with y = mx + C, we find that  Slope m = h (Planck’s constant)