In Young's double-slit experiment, **incoherent** sources (rapidly varying phase difference) give:
ASharp interference fringes
BNo fringe pattern — only an average intensity I = 2 I₀ at every point on the screen
CDestructive interference everywhere
DConstructive interference everywhere
Answer & Solution
Correct answer: B. No fringe pattern — only an average intensity I = 2 I₀ at every point on the screen
**Incoherent sources** (e.g., two separate sodium lamps): phase difference varies rapidly with time → **time-averaged intensity = 2 I₀ uniformly**. **No fringes**. Young's trick was to use a single source illuminating both slits to enforce coherence.
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