Assuming \({W_0}\) to be negligible in comparison to \(\frac{{hv}}{\lambda }\) i.e. \(v_{\max }^2 \propto \frac{1}{\lambda } \)
\(\Rightarrow {v_{\max }} \propto \frac{1}{{\sqrt \lambda }}\). (On increasing wavelength \(\lambda\) to \(4 \lambda\), \(v_{max}\) becomes half).