Diffraction

White light passes through a small hole and spreads out in the focal plane of a lens. Each colour makes its own pattern, the squared magnitude of the hole's Fourier transform, at a scale proportional to its wavelength, red wider than blue; the picture is all of them added together, which is why the fringes away from the centre are coloured. The corner shows the hole itself. Change the shape: a circle gives rings, every edge of a polygon throws out a streak at right angles to itself, slits give a row of fringes that sharpen as slits are added, and a lattice of holes gives a lattice of bright points. Make the hole larger and the pattern shrinks by the same factor.