In YDSE, if the light source has less coherence , the fringes become faint, blurred, and may eventually disappear completely. The reason is that the phase difference keeps changing, so bright and dark bands cannot stay stable on the screen.

What changes

  • Low visibility: The contrast between bright and dark fringes drops.
  • Blurred pattern: The pattern becomes washed out because the interference is no longer steady.
  • No clear fringes: If coherence is too low, a stable interference pattern is not observed.

Why this happens

YDSE needs a fixed phase relation between the two waves reaching the screen. When coherence is poor, that phase relation varies with time, so constructive and destructive interference keep shifting.

Simple way to think about it

Think of it like two singers trying to sing in perfect sync. If their timing keeps drifting, the harmony turns messy and the clear effect disappears. In the same way, low coherence makes the interference pattern unstable.

In exams

A safe one-line answer is: When coherence of the light source is less in YDSE, the interference fringes lose contrast and become indistinct; with very low coherence, the fringe pattern disappears.

TL;DR: less coherence means less stable phase difference, so the YDSE pattern becomes weak, fuzzy, and may vanish.