Which of the following particles has the highest de Broglie wavelength $\lambda$ if they all move with the same velocity $v$?

Solution

1. The de Broglie wavelength is $\lambda =\frac{h}{mv}$.
2. Since $h$ and $v$ are constant, $\lambda \propto \frac{1}{m}$.
3. The electron has the smallest mass among the given particles.
4. Therefore, the electron has the highest wavelength.
5. Hence the answer is (D).