Wave Optics in Physics

Two beams of light meeting in the same place behave in a way no stream of separate particles would. They add, and whether they add to brightness or to darkness depends entirely on how far one has travelled compared with the other. That single idea, the path difference, produces the whole subject. Young's experiment turns it into a row of bright and dark bands whose spacing reads off the wavelength directly, a thin film turns it into the coloured swirls on a soap bubble, and a Michelson interferometer turns it into a distance measurement of startling precision. The blocks give the path difference and the two conditions, the fringe positions and spacing, the intensity pattern, the half wavelength shift that reflection sometimes adds, and the film and interferometer relations that follow from it.