What is the effect of using monochromatic light in Young’s double-slit experiment?

Using monochromatic light—light of a single wavelength and color—has three main effects in Young’s double‑slit experiment:

1. Sharp, high‑contrast fringes
   All the waves have the same wavelength and fixed phase relationship, so the bright and dark bands are crisp and evenly spaced. Nothing blurs the pattern.
2. Uniform fringe spacing and color
   Every fringe appears the same hue and the distance between adjacent bright (or dark) bands stays constant across the screen, making measurements straightforward.
3. No overlapping patterns
   With just one wavelength present, you avoid the multiple, slightly shifted patterns that different colors would create. That prevents color fringes from washing out the visibility of the interference pattern.

Because of these effects, monochromatic light is ideal for clearly demonstrating interference—and for using the pattern to measure the wavelength or slit spacing with precision.