\(\lambda=\frac{\mathrm{h}}{\mathrm{mv}}\)
\(\lambda_{0}=\frac{\mathrm{h}}{\mathrm{m} \sqrt{2} \mathrm{v}_{0}}\dots(1)\)
\(\lambda^{\prime}=\frac{\mathrm{h}}{\sqrt{\mathrm{v}_{0}^{2}+\mathrm{v}_{0}^{2}+\left(\frac{\mathrm{eE}_{0} \mathrm{t}}{\mathrm{m}}\right)^{2}}}\)
\(=\frac{h}{m \sqrt{2 v_{0}^{2}+\frac{e^{2} E_{0}^{2} t^{2}}{m^{2}}}}\dots(2)\)
\(\mathrm{By}(1)\) and ( 2)
\(\lambda^{\prime}=\frac{\lambda_{0}}{\sqrt{1+\frac{\mathrm{e}^{2} \mathrm{E}_{0}^{2} \mathrm{t}^{2}}{2 \mathrm{m}^{2} \mathrm{v}_{0}^{2}}}}\)