JEE Main · 2021 · 24 Feb 2021 · Shift 2 · Wave Optics

The fringe width in a double slit pattern is $\beta =\frac{\lambda D}{d}$, directly proportional to the wavelength. Violet light has the shorter wavelength, about $400$ nm against roughly $700$ nm for red, so with the same geometry $\beta$ falls to a little over half its earlier value. The central fringe stays bright, because zero path difference is independent of wavelength, and the minima stay perfectly dark, because the two beams still arrive with equal amplitudes. Consecutive fringe lines will come closer together.