At zero angle every slit element is in <phase> for every <wavelength>. Their central maxima coincide, so \b[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.

Solution

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

 Solution (source code)