The stopping potential is greater for sodium as it is easier to ionise sodium, than copper.
[Rhc=13.6 eV, $\mathrm{hc}=1242 \mathrm{eV} \mathrm{nm}, \mathrm{h}=6.6 \times 10^{-34} \mathrm{J-s}$ અને હાઈડ્રોજન પરમાણુનું દળ $\sim 1.6 \times 10^{-27} \mathrm{~kg}$ લો.]