White light consists of waves of inumerable wavelengths starting from violet to red colour. Therefore if 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 pointMof screenS1m− S2m=0

i.e., the waves of all colours reach at mid pointMin same phase. Therefore the central fringe (atM) iswhite. As fringe width ,i.e., wavelength increases in 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.