A photon of wavelength $1000 \stackrel{\mathrm{o}}{\mathrm{A}}$ has energy $12.3 \mathrm{eV}$. If light of wavelength $5000 \stackrel{\mathrm{o}}{\mathrm{A}}$, having intensity I, falls on a metal surface, the saturation current is $0.40 \mathrm{\mu A}$ and the stopping potential is $1.36 \mathrm{V}$. The work function of the metal is

Stopping potential of $1.36 \mathrm{V}$ implies kinetic energy of photoelectrons ${\left(K.E.\right)}_{max}=1.36 \mathrm{eV}$.

If a photon of wavelength $1000 \stackrel{\mathrm{o}}{\mathrm{A}}$ has energy 12.3 eV, energy of a photon of wavelength $5000 \stackrel{\mathrm{o}}{\mathrm{A}}$ is

$E=12.3\times \frac{1000}{5000}=2.46 \mathrm{eV}$.

Work function of the metal  ${\varphi }_0=E-{\left(K.E.\right)}_{max}=2.46-1.36=1.1 \mathrm{eV}$