Given :-

Work function of tantalum = 4.2 eV.
Work function of barium = 2.5 eV.
Work function of lithium = 2.3 eV.

We know that, visible light range is from 4000 A⁰ to 7000 A⁰

Energy of visible light photon of wavelength 4000 A⁰
= h (freq)
= h c / (lamda)
= [(6.6 x 10^-34 J.s) (3 x 10⁸ m / s)] / (4000 x 10^-10 m)
= 4.95 x 10^-19 J.
= [(4.95 x 10^-19) / (1.6 x 10^-19)] eV
= 3.09 eV

Energy of visible light photon of wavelength 7000 A⁰
= h (freq)
= h c / (lamda)
= [(6.6 x 10^-34 J.s) (3 x 10⁸ m / s)] / (7000 x 10^-10 m)
= 2.83 x 10^-19 J.
= [(2.83 x 10^-19) / (1.6 x 10^-19)] eV
= 1.77 eV

So, for a potocell operable with visible light, the work function of the substance (i.e. photosensitive cathode) must lie in the range from 1.77 eV to 3.09 eV. Thus we can choose, Barium with work function 2.5 eV & Lithium with work function 2.3 eV as photocell materials for visible light.

==============
By :-   Dr. A. W. Pangantiwar