The Michelson interferometer splits a light beam with a semi-transparent mirror, sends each part on a different round-trip path, and recombines them to make interference fringes that reveal phase differences. Because a fringe shift corresponds to only a fraction of a wavelength, the setup can detect changes in length smaller than a nanometre. Michelson used it to re-measure the speed of light, reducing the uncertainty to about 0.02 %. In spectroscopy, the same instrument let him compare spectral lines at extremely high resolution, greatly improving elemental identification. These achievements underpin modern technologies such as laser-interferometric gravitational-wave observatories and nanometre-accurate positioning in semiconductor fabrication.

In a Michelson interferometer, a path difference ΔL produces a phase shift Δφ=2π(2ΔL)/λ, and the detected intensity I=I0[1+cos Δφ] allows ΔL to be inferred with better than λ/2 precision. Michelson combined diffraction-limited optics with a green mercury line (λ≈546 nm) to propose a vacuum-based length standard. For his speed-of-light measurement he enclosed a ~600 m folded path in low-pressure tubes and used a rapidly rotating mirror to convert the travel time into an angular deflection. The resulting value, 299 774 ± 11 km s⁻¹, served as the world standard until mid-century. The interferometer concept was later generalized for refractive-index determination, thin-film thickness control, and infrared wavelength standards, becoming a cornerstone of metrology.

The Michelson interferometer, a two-beam system without a Zernike phase plate, achieved picometre-level differential displacement sensitivity despite coherence-length limitations. Michelson synchronously drove a multi-facet rotating mirror at 528 rps, attaining a temporal resolution of 10⁻⁸ s along an effective 30 km round-trip path. For spectral work he employed fringe counting with a vertical comparison arm, reporting doublet separations with Δλ/λ≈10⁻⁷ for several elements. These methods accelerated the shift from the platinum–iridium metre artifact toward optical wavelength standards and ultimately the CODATA2019 fixed definition of c. Present-day Fabry–Pérot-stabilized lasers and the power-recycled LIGO/Virgo interferometers are direct descendants of Michelson’s architecture.