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Question:

A parallel beam of fast moving electrons is incident normally on a narrow slit. A fluorescent screen is placed at a large distance from the slit. If the speed of the electrons is increased, which of the following statement is correct?

The angular width of the central maximum will remain unaffected

Diffraction pattern is not observed on the screen in the case of electrons

The angular width of the central maximum of the diffraction pattern will increase

The angular width of the central maximum will decreases

Solution:

We know that λ=h/mv. Also for angular width θ=2λ/d. So we can say θ ∝ λ ∝ 1/v. So if the velocity increases, the angular width of the central maximum will decrease.