An electron is moving with initial velocity v = ak in a magnetic field B = bj. Its de-Broglie wavelength will (a) decrease and increase periodically. (b) increase with time. (c) decrease with time. (d) remain constant.

Answer: (d). v along k and B along j: magnetic force F = -e( v× B) is always perpendicular to v. It does no work, so speed and |p| stay constant. λ = h/p stays constant. So the answer is (d) remain constant.

An electron is moving with initial velocity v = ak in a magnetic… | MHT CET 2026 (12 May, Shift 1)

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Matter Waves: de Broglie Wavelength

An electron is moving with initial velocity $\vec{v} = a\hat{k}$ in a magnetic field $\vec{B} = b\hat{j}$ . Its de-Broglie wavelength will

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Answer: (d)

$\vec v$ along $\hat k$ and $\vec B$ along $\hat j$ : magnetic force $\vec F = -e(\vec v\times\vec B)$ is always perpendicular to $\vec v$ .
It does no work, so speed and $|p|$ stay constant.
$\lambda = h/p$ stays constant.
So the answer is (d) remain constant.

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