Coherence And Interference Conditions

Sustained interference, according to NCERT, requires two coherent sources, meaning they must emit waves of the same frequency and maintain a constant phase difference over time. Ordinary bulbs emit light in random, rapidly changing bursts from countless independent atoms, so the phase relationship between two separate bulbs fluctuates far too quickly for any steady fringe pattern to build up. This is why a single source split into two, as in Young's experiment, is used to guarantee coherence. The option blaming dimness is wrong because brightness affects visibility of fringes but not their existence; the fundamental issue is coherence. The option requiring three sources is wrong because two coherent sources are entirely sufficient, as Young demonstrated. The option about wall absorption is wrong because ordinary walls scatter light without preventing interference in principle. As stated in NCERT Class 12, Chapter 10, only coherent sources with a fixed phase relationship can produce a stable interference pattern. A consistency check confirms that splitting one beam ensures a locked phase, which is precisely how real interference experiments succeed.