The stopping potential V for photoelectric emission from a metal surface is plotted along Y-axis and frequency v of incident light along X-axis. A straight line is obtained as shown in the figure. Planck’s constant is given by:

product of slope of the line and charge on the electron

According to Einstein’s photoelectric equation

$K=eV=hv-W_0$ or $V=\frac{h}{e}v-\frac{W_0}{e}$

∴ Slope of straight line between V and v is $\frac{h}{e}$.

∴ h = e × slope of straight line.