Fringe spacing formula in double-slit interference

Steps:

• The fringe separation β is given by β = λD / d, where λ is wavelength, D is screen distance, and d is slit separation.
• To increase β, decrease d while keeping λ and D constant.
• Option A decreases d, directly increasing β.
• Options B, C, and D either do not affect d or change λ/D to reduce β.

Why A is correct:

• The double-slit interference formula β = λD / d shows that decreasing slit separation d increases fringe spacing β.

Why the others are wrong:

• B: Slit width affects diffraction envelope width but not interference fringe spacing β.
• C: Decreasing D reduces β per the formula.
• D: Higher frequency shortens λ, reducing β per the formula.

Final answer: A