Why can't the headlights of two different cars ever produce an interference pattern?
Interference needs coherent sources — a constant phase relationship.
Interference needs coherent sources — a constant phase relationship. Two independent bulbs (or headlights) emit light in tiny random bursts with phases that change millions of times a second, so any pattern would wash out instantly. That's why Young split light from ONE source into two slits: only then are the two beams coherent and the fringes stable.
Key point
Steady interference needs coherent (constant-phase) sources, so one source is split in two.
Common mistake
expecting any two light sources to interfere.
Memory tip
no coherence, no fringes — split one source instead of using two.
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