(a) (i) Photoelectric emission takes place when $\mathrm{mathrm}\mathit{Z}\mathit{n}$ is irradiated with UV radiation.
(ii) Photoelectric emission takes place when $\mathrm{mathrm}\mathit{N}\mathit{a}$ is irradiated with visible light.(b) Maximum kinetic energy of photoelectron
$=$ Energy of incident photon - Work function
Or, $6.06\times {10}^{-19}=\mathit{h}\mathit{v}-4.5\times 1.6\times {10}^{19}$
Or, $\mathit{h}\mathit{v}=13.26\times {10}^{-19}$
Or, $\mathit{v}=\frac{\left(13.26\times {10}^{-19}\right)}{\left(6.6\times {10}^{-34}\right)}$
$\therefore \mathit{v}=2\times {10}^{15}\mathit{H}\mathit{z}$