Exuberant interference: rainbows, tides, edges, (de)coherence ...

Young's pioneering studies of interference have led to fundamental developments in wave physics. Supernumerary rainbows were the first example of diffraction associated with caustics. Cotidal lines (connecting places where the tide is high at a given time) were the first example of wavefronts in the modern sense (pattern of phase contours (arg psi1 + psi2) of the superposition of waves psi1 and psi2, rather than the superposed patterns of the separate phases arg psi1 and arg psi2), and led to the discovery of phase singularities. Edge-diffracted waves extend the range of asymptotic methods applied to waves and continue to find diverse and unexpected applications. Young's understanding of the conditions for observing interference are now part of decoherence theory, which explains, for example, the emergence of the classical world from the quantum world.