Question

Sol. [Key Concept: Photoelectric effect — threshold wavelength from work function] For incident radiation having wavelength $(\lambda)$, photoelectric effect doesn't occur when $\frac{hc}{\lambda} <$ work-function

$$\begin{matrix} & \ \Rightarrow \lambda > \frac{hc}{W_{0}} \\ & \ \Rightarrow \lambda > \frac{6.6 \times 10^{- 34} \times 3 \times 10^{8}}{6.6 \times 1.6 \times 10^{- 19}} \\ & \ \Rightarrow \lambda > \frac{3 \times 10^{- 7}}{1.6} \\ & \ \Rightarrow \lambda > \frac{300}{1.6}\text{ }nm \\ & \ \Rightarrow \lambda = 187.5\text{ }nm \end{matrix}$$

∴ Option (3) 200 nm is correct.