Fringe separation formula: β = λD / d

Steps:

• Recall fringe separation β = λD / d, where λ is wavelength, D is screen distance, d is slit separation.
• To increase β, increase λ, increase D, or decrease d.
• Evaluate options: A affects slit width (diffraction envelope, not β); B decreases D (reduces β); C increases d (reduces β); D increases λ (increases β).
• Select D, as red light's longer wavelength directly enlarges fringes.

Why D is correct:

• Red light has longer wavelength (λ) than green; β ∝ λ, so separation increases per interference formula.

Why the others are wrong:

• A: Wider slits broaden diffraction pattern but leave interference spacing β unchanged.
• B: Nearer screen decreases D, so β ∝ D reduces separation.
• C: Wider slit separation increases d, so β ∝ 1/d reduces separation.

Final answer: D