\(\lambda=6630 \; \mathring A\)
\(E =\phi+ eV _{0}\)
\(E\) : energy of incident photon
\(V_{0}:\) Stopping potential
\(\phi= E - eV _{0}\)
\(E =\frac{12400}{6630} eV =1.87 \; eV\)
\(\phi=(1.87-0.42)=1.45 \; eV\)
\(\phi= hv _{0} ; v _{0}\) : threshold frequency
\(1.45 \times 1.6 \times 10^{-19}=6.63 \times 10^{-34} \times v _{0}\)
\(v _{0}=0.35 \times 10^{15}\)
\(=35 \times 10^{13} sec ^{-1}\)
\(=35\)