The work function for caesium atom is 2 eV,  the threshold wavelength in nm is

work function W0=2 eV=2×1.6×10−19J =3.2×10−19J W0=hv0=hcλ0 ∴ λ0=hcW0=6.626×10−34Js×3×108ms−13.2×10−19J =6.5388×10−7 m =653.88nm