In a sample of excited hydrogen atoms electrons make transition from $n=2$ to $n=1.$ Emitted photons strike on a metal of work function $\left(\varphi \right)4.2eV.$ Calculate the wavelength (in $\stackrel{\text{o}}{\text{A}}$ ) associated with ejected electrons having maximum kinetic energy. $(\sqrt{174.9}=13.2)$