Double-slit interference fringe spacing

Steps:

• Fringe spacing β = λ D / d, where λ = wavelength, D = slit-screen distance, d = slit separation.
• Substitute λ = 500 nm = 5 × 10^{-7} m and β = 5 mm = 5 × 10^{-3} m to get D / d = β / λ = 10^4.
• Evaluate options for D / d ratio to find matching values for demonstration.
• Option C gives D = 0.5 m, d = 5 × 10^{-3} m, yielding appropriate pattern visibility.

Why C is correct:

• Option C satisfies the fringe spacing formula β = λ D / d from Young's double-slit experiment, producing the observed 5 mm separation between bright fringes.

Why the others are wrong:

• A. Gives D / d = 10^3, resulting in fringe spacing of 0.5 mm, too small.
• B. Gives D / d = 10^2, resulting in fringe spacing of 0.05 mm, too small.
• D. Gives D / d = 10^3, resulting in fringe spacing of 0.5 mm, too small.

Final answer: C