Physics · Wave Optics · NEET
The central point is exactly equal distance from both slits, so the path difference there is zero. A path difference of zero means constructive interference (a bright fringe) for EVERY wavelength at the same spot. Red, green, violet - all colours are bright together at the centre. When all colours of white light overlap and are bright together, our eye sees white. So the centre is white.
Away from the centre, a bright fringe forms where path difference = n times lambda. Because fringe width beta = D lambda / d depends on wavelength, each colour has its own fringe spacing. Violet (small lambda) has closely packed fringes, red (large lambda) has wider spaced fringes. So the first-order bright fringes of different colours fall at slightly different places, and instead of overlapping into white they separate into colours. That is why you see coloured fringes beside the white centre.
Violet is nearest to the centre. Since fringe position y = n D lambda / d, a smaller wavelength gives a smaller y (closer to the centre). Violet has the smallest wavelength (about 400 nm), so its first bright fringe sits closest to the white centre. Red has the largest wavelength (about 700 nm), so red is farthest out. So each coloured fringe runs violet (inner) to red (outer).
As you go far from the centre, the higher order fringes (n = 3, 4, 5...) of many different colours start to overlap each other. A bright fringe of one colour lands on top of a dark region of another colour and a bright fringe of a third colour, and so on. So many colours pile up at every point that they mix back into white again. This uniform white region has no clear fringes - the interference pattern is said to wash out.
Yes, at the exact centre it is still bright for both colours because path difference is zero for both. But it will not look pure white - it will look like the mix of only those two colours (for example red plus green looks yellowish). True white needs the full band of colours. The key idea stays the same: zero path difference is bright for all wavelengths present.
If the monochromatic source in Young's double slit experiment is replaced by white light, then
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is bright and white. Path difference is zero at the centre, which is the condition for a bright fringe for every colour, so all colours are bright together and give white.
From the white centre outward each fringe goes violet first (smallest wavelength, closest) to red last (largest wavelength, farthest). This repeats for each order until the colours overlap and wash out.
Monochromatic light is a single colour, so its central fringe is that same colour (for example red light gives a red central fringe). White light contains all colours, and because zero path difference makes all of them bright together at the centre, they mix into white.
The formula beta = D lambda / d is the same, but lambda is different for each colour. So each colour has its own fringe width. The centre is common to all colours (zero path difference), which is why only the centre is white and the sides split into colours.
