Single-slit diffraction pattern

Steps:

• In diffraction through a gap of width a = λ, the central maximum spreads over angle θ where sinθ ≈ λ/a = 1, so θ ≈ 90°.
• Widening the gap increases a while keeping incident wavelength λ fixed.
• The diffraction angle θ decreases as θ ∝ λ/a; larger a narrows the spread.
• Emerging waves propagate in the same medium, so their wavelength equals the incident λ and remains unchanged.

Why A is correct:

• θ decreases with increasing gap width per the diffraction formula sinθ = mλ/a (m=1 for first minimum), while both wavelengths stay constant in the same medium.

Why the others are wrong:

• B: θ increases is incorrect; wider gap reduces diffraction spread.
• C: θ unchanged is wrong; angle depends on a/λ ratio.
• D: θ unchanged and incident λ increases contradict fixed source wavelength.

Final answer: A