The Michelson Interferometer

Albert Michelson built it in 1881 to detect the aether and it failed at that, which is the least interesting thing about it. What he had actually made was the first instrument that could compare any length at all against the wavelength of light. A beamsplitter divides the light in two. One half goes to a fixed mirror, the other to a mirror you can move, and both come back to be recombined. Move the moving mirror by half a wavelength — about three hundred nanometres — and its arm's path changes by a full wavelength, and the fringes advance by exactly one. Count the fringes and you have measured the movement. Nothing touches anything, nothing wears, and the ruler is a property of the atom that emitted the light. The 1887 experiment with Edward Morley is famous for finding nothing, and the null was emphatic rather than marginal: the apparatus could have seen the predicted effect forty times over. It sat unexplained until Einstein made it unsurprising in 1905. Michelson took the 1907 Nobel Prize — the first to an American scientist — and the citation was for the instrument. FULLY BUILDABLE, and it is the one to build. A beamsplitter, two front-surface mirrors and a laser on a sandbox will resolve a movement of three hundred nanometres. Breathe near one arm and the fringes sweep, because warm air has a different refractive index and you have just measured that too.