The threshold frequency for photo electric effect on sodium corresponds to a wavelength 5000 Å. Its work function is

15 Joule 10 × 10–19 Joule 4 × 10–19 Joule None of these

4 × 10–19 Joule

$\phi=\frac{hc}{λ_0}$ $\phi=\frac{20×10^{-26}}{5000×10^{-10}}=4×10^{-19}Joule$