Interferometry is a high-precision optical measurement technique built on the wave nature of light. When two beams of coherent, monochromatic light (from a laser or a spectral lamp) are made to overlap, they interfere: where the waves arrive in phase they reinforce to give bright bands, and where they arrive out of phase they cancel to give dark bands. These alternating bright and dark bands are called interference fringes, and each fringe corresponds to a path difference of one half or one full wavelength of light — a distance of only a few hundred nanometres.

Because a fringe maps to such a tiny change in optical path, interferometry can resolve surface variations far smaller than mechanical instruments can. Its primary metrological application is measuring surface flatness with very high (sub-micron / nanometre-level) precision. In a typical setup an optical flat is placed over the workpiece; light reflecting from the reference flat and from the test surface interferes, and the pattern of fringes reveals the surface geometry:

• Straight, evenly spaced, parallel fringes indicate a truly flat surface.
• Curved or unevenly spaced fringes reveal hollows, high spots or a wedge, and counting the fringes quantifies the departure from flatness.

This technique is also the basis for calibrating slip gauges and precision length standards. The other options belong to unrelated instruments and principles. Analysing thermal conductivity uses heat-flow methods, not optics. Determining chemical composition by spectroscopy analyses the wavelengths a substance emits or absorbs — a different optical technique that identifies elements, not surface geometry. Calculating torque in shafts is done with strain gauges or torque sensors. None of these measure form, so the correct application is high-precision measurement of surface flatness.