When a metallic surface is illuminated with monochromatic light of wavelength λ, stopping potential for photoelectric current is 3 V₀. When the same metallic surface is illuminated with a light of wavelength 2λ, the stopping potential is V₀. The threshold wavelength for the surface is-

4λ

$3V_0=\frac{hc}{eλ}-\frac{hc}{eλ_0}$  ...(1)

$V_0=\frac{hc}{2eλ}-\frac{hc}{eλ_0}$  ...(2)

$\frac{eq^n(1)}{eq^n(2)}⇒\frac{3V_0}{V_0}=\frac{\frac{hc}{eλ}-\frac{hc}{eλ_0}}{\frac{hc}{2eλ}-\frac{hc}{eλ_0}}$

$\frac{3}{2}\frac{hc}{eλ}-\frac{3hc}{eλ_0}=\frac{hc}{eλ}-\frac{hc}{eλ_0}$

$\frac{3}{2λ}-\frac{3}{λ_0}=\frac{1}{λ}-\frac{1}{λ_0}$

$\frac{2}{λ_0}=\frac{1}{2λ}$

$λ_0=4λ$