Newton's Rings

Hooke saw them in 1665 and Newton measured them, which is the difference that matters. Press a convex lens onto a flat piece of glass and coloured rings appear around the contact point — in monochromatic light, dark and bright circles; in daylight, a bullseye of spectral colour. The wedge of air between the two surfaces grows in thickness as the square of the distance from the contact. Light reflected from the top of that wedge and from the bottom travels different distances and interferes, so a ring appears wherever the extra path is a whole number of wavelengths. Count the rings and you have measured the gap — in units of half a wavelength, about a third of a micrometre, on apparatus that costs less than a set of drill bits. The centre is DARK in reflection, and that detail defeated Newton. His corpuscles offered no reason why two surfaces in contact should reflect nothing at all. The wave explanation is one line: reflection off the denser medium at the bottom of the film flips the phase, the top reflection does not, and at zero thickness the two cancel exactly. He recorded the effect faithfully for forty years and never could account for it. This is the first blueprint in an interferometry batch because it is the whole idea in its simplest form: stop comparing your object to a ruler and start comparing it to a wave.