Superposition of two-wavelength interference patterns

Steps:

• Single slit source emits coherent red and violet light to the double slit.
• Each wavelength creates fringes with spacing β = λD/d; red (longer λ) has wider spacing than violet.
• Patterns overlap on the screen, causing intensity modulation from constructive/destructive interference of the two sets.
• Resulting pattern shows beats or varying contrast due to slight wavelength difference.

Why C is correct:

• C depicts modulated fringes from superposition of two patterns with similar but distinct spacings, per wave interference principle.

Why the others are wrong:

• A shows uniform single-wavelength fringes, ignoring dual colors.
• B illustrates separated patterns, but overlaps occur on one screen.
• D represents no interference, just broad illumination.

Final answer: C