When light waves of differing frequencies are made incident on the same surface, the stopping potential ($V_s$) differ as is evidenced from Einstein's photoelectric equation given by

$hv-h{v}_0=e{V}_s$

where $h$ is Planck's constant, $v,v_0$ the frequencies and $e$ the charge.



Hence, ${ V}_s=\frac{hv}{e}-\frac{hv}{e}$ ...(i)

Comparing Eq. (i) with straight line equation

$y=mx+c$

Slope $m=\frac{h}{e}$

$Where h=6.6\times {10}^{-34} Js, e=1.6\times {10}^{-19} C.$

$\therefore m=\frac{6.6\times {10}^{-34}}{1.6\times {10}^{-19}}=4.125\times {10}^{-15}$