An electron with mass ' m ' with an initial velocity $(t=0) \vec{v}=v_{0} \hat{i}\left(v_{0}>0\right)$ enters a magnetic field $\vec{B}=B_{0} \hat{j}$. If the initial de-Broglie wavelength at $\mathrm{t}=0$ is $\lambda_{0}$ then its value after time ' t ' would be :
