Fringe separation in double-slit interference

Steps:

• Recall the fringe separation formula: Δy = λL / d, where λ is wavelength, L is screen distance, and d is slit separation.
• To increase Δy, increase λ, increase L, or decrease d.
• Evaluate options: A decreases L (reduces Δy); B decreases d (increases Δy); C and D use shorter wavelengths than red (reduce Δy).
• Confirm B as the only change that increases separation.

Why B is correct:

• Decreasing slit separation d directly increases Δy per the interference formula Δy = λL / d.

Why the others are wrong:

• A: Decreasing L reduces Δy proportionally.
• C: Blue light has shorter λ than red, reducing Δy.
• D: Green light has shorter λ than red, reducing Δy.

Final answer: B