Case \((ii)\), \(e V_{0}=\frac{h c}{2 \lambda}-\phi_{0}\)     .........\((ii)\)

From \((i)\) and \((ii)\) we get,

\(\frac{3 h c}{2 \lambda}-3 \phi_{0}=\frac{h c}{\lambda}-\phi_{0}\)

or \(\frac{3 h \lambda}{2 \lambda}-\frac{h c}{\lambda}=3 \phi_{0}-\phi_{0}\)

or \(\frac{h c}{2 \lambda}=2 \phi_{0}\) or \(\phi_{0}=\frac{h c}{4 \lambda}\)

\(\therefore \) Threshold wavelength

\(\lambda_{0}=\frac{h c}{\phi_{0}}=\frac{h c}{h c} \times 4 \lambda=4 \lambda\)