Fringe Width in Double-Slit Interference

Steps:

• The fringe separation β is given by β = λD / d, where λ is wavelength, D is source-to-screen distance, and d is source separation.
• Increasing D directly increases β proportionally.
• Increasing d decreases β.
• Increasing frequency f decreases λ (λ = c/f), so β decreases.
• Intensity affects amplitude but not β.

Why A is correct:

• From β = λD / d, increasing D enlarges fringe separation as it scales linearly with screen distance.

Why the others are wrong:

• B: Larger d reduces β by increasing denominator in the formula.
• C: Higher frequency shortens λ, shrinking β.
• D: Intensity changes brightness but not spatial pattern or β.

Final answer: A