Fringe separation proportional to screen distance

Steps:

• Fringe width β = λD/d, so β ∝ D for fixed λ and d.
• Initial D1 = 0.6 m, β1 = 1.8 mm.
• New D2 = 0.6 m + 0.9 m = 1.5 m.
• β2 = β1 × (D2/D1) = 1.8 mm × (1.5/0.6) = 1.8 mm × 2.5 = 4.5 mm.

Why C is correct:

• Matches β ∝ D from double-slit interference formula, yielding exactly 4.5 mm.

Why the others are wrong:

• A: Assumes inverse proportionality, halving separation incorrectly.
• B: Uses 1.5× factor instead of 2.5×, underestimating increase.
• D: Approximates 1.8×3=5.4 mm but ignores exact 2.5 ratio.

Final answer: C