Question

What kind of fringes do you expect to observe if white light is used instead of monochromatic light?

Answer 1

White light consists of waves of all wavelengths starting from violet to red colour. If the monochromatic light in Young’s interference experiment is replaced by white light, then the waves of each wavelength form their separate interference patterns. The resultant effect of all these patterns is obtained on the screen.

The path difference between waves starting from S₁ and S₂  at the location (M) of central fringe is zero, i.e., for point M of screen S₁M − S₂M=0  i.e., the waves of all colours reach at midpoint M in the same phase. Therefore the central fringe (at M) is white. As fringe width ω = Dλ/d or ω ∝ α and in visible region wavelength of violet colour is least and that of red colour is maximum, i.e., wavelength increases in the order of colours denoted by VIBGYOR therefore on either side of it some coloured fringes are obtained in order of colour VIBGYOR. That is the violet (V) fringe appears first and the red (R) the last. After this, the fringes of many colours overlap at each point of the screen and so the screen appears uniformly illuminated.

Thus if we use white light in place of monochromatic light the central fringe is white, containing on either side a few coloured fringes (in order VIBGYOR) and the remaining screen appears uniformly illuminated.