When light waves of differing frequencies are made incident on the same surface, the stopping potential ( Vs ) differ as is evidenced from Einstein's photoelectric equation given by hv−hv0=eVs
where h is Planck's constant, v,v0 the frequencies and e the charge.
Hence, Vs=ehv−ehv ...(i)
Comparing Eq. (i) with straight line equation y=mx+c
Slope m=eh Whereh=6.6×10−34Js,e=1.6×10−19C. ∴m=1.6×10−196.6×10−34=4.125×10−15