AS Daily A Level Physics question
In a lab, a narrow electron beam is adjusted so that the electrons move at twice their previous speed, with everything else unchanged. Which statement must be true about the electrons’ de Broglie wavelength?
Answer
The correct answer is C.
Correct: C — Becomes half its original value. The de Broglie wavelength is inversely proportional to particle speed for fixed mass, so doubling speed halves the wavelength (a simple 1:2 ratio). A Decreases by a factor of four suggests a 1/v^2 dependence, which is a misconception. B Increasing to twice its original value reverses the actual inverse relationship (many think faster means a longer wavelength, but it’s the opposite). C This is correct because λ ∝ 1/v, so doubling v → λ/2. D No change ignores the dependence of wavelength on momentum/speed.