Fringe spacing in double-slit interference depends on wavelength, slit separation, and screen distance

Steps:

• Recall the fringe spacing formula: β = λD / d, where λ is wavelength, D is slit-to-screen distance, and d is slit separation.
• To increase β, increase λ or D, or decrease d.
• Evaluate each change: decreasing λ reduces β; decreasing D reduces β; decreasing d increases β.
• Identify which distance corresponds to d based on the effect that increases β.

Why B is correct:

• Decreasing r reduces slit separation d, so β = λD / d increases per the interference formula.

Why the others are wrong:

• A: Decreasing q reduces screen distance D, so β decreases.
• C: Decreasing wavelength λ directly reduces β.
• D: Decreasing p likely reduces another parameter like D or has no increasing effect on β.

Final answer: B