Fringe spacing proportional to wavelength

Steps:

• Fringe spacing β = λD/d; D and d unchanged, so β ∝ λ.
• Ratio of wavelengths: λ_blue / λ_red = 4.5 × 10^{-7} / 7.0 × 10^{-7} = 4.5/7.0 ≈ 0.643.
• New spacing: β_blue = 3.5 mm × 0.643 ≈ 2.25 mm, rounds to 2.3 mm.
• Matches option A.

Why A is correct:

• β ∝ λ from double-slit formula; shorter blue wavelength yields smaller spacing.

Why the others are wrong:

• B: Same spacing ignores wavelength change.
• C: Larger than original, contradicts shorter λ.
• D: Inverse ratio (7/4.5 × 3.5 ≈ 5.4 mm, not 9); wrong proportionality.

Final answer: A