(a) Reason
(b) Formula
(c) Calculation of range of wave length.
Reason:
Under reverse bias change in current with the change in intensity of light is more significant.
Calculation:
$\mathit{\lambda }=\frac{\mathit{h}\mathit{c}}{\mathit{E}}$
${\mathit{\lambda }}_1=\frac{6.63\times {10}^{-34}\times 3\times {10}^8}{2.5\times 1.6\times {10}^{-19}}$ $=497\times {10}^{-9}\mathrm{m}$
${\mathit{\lambda }}_2=\frac{6.63\times {10}^{-34}\times 3\times {10}^8}{2.5\times 1.6\times {10}^{-19}}$ $=444\times {10}^{-9}\mathrm{m}$
Range is $444\times {10}^{-9}\mathrm{m}$ to $497\times {10}^{-9}\mathrm{m}$