Monochromatic radiation of wavelength $640.2\: nm$ from a neon lamp irradiates photosensitive material made of cesium on tungsten. The stopping voltage is measured to be $0.54\: V$. The source is replaced by an iron source and its $427.2\: nm$ light irradiates then same photocell. Predict the new stopping voltage.

Question

$\begin {array} {1 1} (a)\;2.29\: eV & \quad (b)\;1.39\: eV \\ (c)\;1.5\: eV & \quad (d)\;2.4\: eV \end {array}$