Answer
The de Broglie wavelength remains the same.
Work Step by Step
We can use Equation (38-20) to find an expression for the de Broglie wavelength:
$\frac{1}{2}mv^2 = \frac{h^2}{2m \lambda^2}$
$\lambda^2 = \frac{h^2}{m^2~v^2}$
$\lambda = \frac{h}{m~v}$
Since the electron is moving in the same direction as the magnetic field, the magnetic force on the electron is zero. The speed does not change.
Therefore, the de Broglie wavelength remains the same.
