An electron moving with a speed $y_{0}$ along the positive $x$ -axis
at $\mathrm{y}=0$ enters a region of uniform magnetic field $\mathrm{B}^{-}=-\mathrm{B}_{0} \mathrm{k} \wedge$ which exists to the right of y-axis. The electron exits from the region after some time with the speed at co-ordinate y then.