Monochromatic light of frequency f₁ is incident on a photo cell and the stopping potential is found to be V₁. What is the new stopping potential of the cell if it is radiated by monochromatic light of frequency f₂?

$V_1+\frac{h}{e}(f_2-f_1)$

$eV_1 = hf_1 – \phi$

$eV_2 = hf_2 – \phi$

$e(V_1 – V_2) = h (f_1 – f_2)$

$V_1 – V_2 =\frac{h}{e}(f_1 – f_2)$

$V_2 =V_1 +\frac{h}{e}(f_2 – f_1)$