d
\(\frac{1}{2}m\upsilon _{\max }^2\,\, = \,\,\frac{{hc}}{\lambda }\,\, - \,\,\phi \,\,........\,\,(1)\) હવે

\(,\,\,\,\lambda '\,\, = \,\,\lambda \,\, - \,\,0.25\lambda \,\, = \,\,0.75\,\lambda \,\upsilon '\,\, = \,\,2\upsilon \)

\(\therefore \,\,\frac{1}{2}m{(2{\upsilon _{\max }})^2}\,\, = \,\,\frac{{hc}}{{0.75}}\,\, - \,\,\phi \,\,......\,\,(2)\)

સમીકરણ \(\,{\rm{(1)}}\)અને\(\,{\rm{(2)}}\,\) પરથી

\({\rm{4}}\left( {\frac{{\lambda c}}{\lambda }\,\, - \,\,\phi } \right)\,\, = \,\,\frac{{4hc}}{{3\lambda }}\,\, - \,\,\phi \)

\(\,\,\therefore \,\,\frac{{8hc}}{{3\lambda }}\,\, = \,\,3\phi \)

\(\,\therefore \,\,\phi \,\, = \,\,\frac{{8hc}}{{9\lambda }}\) પરંતુ 

\(\frac{{{\rm{hc}}}}{\lambda }\,\, = \,\,2e\,V\,\,\) હોવાથી \(\,\phi \,\, = \,\,\frac{{\rm{8}}}{{\rm{9}}}\,\, \times \,\,2\,eV\,\, = \,\,\frac{{16}}{9}\,\)

\(\,\therefore \,\,\phi \,\, = \,\,1.8\,eV\)