Fringe spacing in double-slit interference

Steps:

• Recall fringe spacing formula: β = λD/d, where λ is wavelength, D is slit-to-screen distance, d is slit separation.
• For red light, λ is larger (~700 nm), producing wider fringes.
• Changing to blue light reduces λ (~450 nm), so β decreases, reducing bright line spacing.
• Other factors like source distance or intensity don't affect spacing directly.

Why A is correct:

• Blue light has shorter wavelength than red, so fringe spacing β decreases per β = λD/d.

Why the others are wrong:

• B: Source-to-slit distance doesn't affect interference pattern on screen.
• C: Slit distance remains 0.5 mm, so no change in β.
• D: Intensity affects brightness, not spacing of fringes.

Final answer: A