Past exam of the physics course of the year 1 of the natural sciences course of the University of Cambridge / 2013 / Paper 1 / Section D / D12 / a / Solution

At zero angle every slit element is in phase for every wavelength. Their central maxima coincide, so the centre is bright white. Shorter wavelengths have narrower central maxima; farther from the axis their contributions fade sooner, giving the central band reddish outer edges for a smooth white spectrum.