The work functions for sodium and copper are 2eV and 4eV. Which of them is suitable for a photocell with 4000 ∘A light
Copper
Sodium
Both
Neither of them
Threshold wavelength for Na, λNa=123752=6187.5 ∘A Also λCu=123754=3093.75 Since λNa>4000∘A; So Na is suitable.