In a photoelectric experiment ultraviolet light of wavelength 280 nm is used with lithium cathode having work function $\phi=2.5 \mathrm{eV}$. If the wavelength of incident light is switched to 400 nm , find out the change in the stopping potential. ( $\mathrm{h}=6.63 \times 10^{-34} \mathrm{Js}, \mathrm{c}=3 \times 10^8 \mathrm{~ms}^{-1}$ )