Interferometry is a
measurement technique that uses the interference of waves. The
wave nature of
photons is most commonly used for interferometry, but as
DeBroglie informed us,
all matter has a wave nature too. Particles with mass (such as
electrons) make effective interferometric specimens as well.
Remember that light is composed of waves of oscillating electric and magnetic fields travelling through spacetime in a vector fashion. The wavelength of these fields for a given photon is constant, and is determined by the energy of the photon. The higher the energy of the photon, the shorter the wavelength of the photon's associated fields.
In order to observe interference between photons, it helps to have them all be of the same wavelength. It also helps if they are coherent. Lasers are designed to emit light satisfying both of these criteria, and are most often used in the construction of the Michelson Interferometer.
Where light of opposite phases meets, destructive interference occurs. Where light of the same phase meets, constructive interference occurs. The design of interferometers is such that by measuring the patterns of constructive and destructive interference, one can measure properties of materials under study. The dimensions of materials are the most commonly interferometrically measured properties. Properties such as absorbance are also measured using interferometric spectroscopy.
Interferometers are capable of measuring all sorts of properties, in all manner of dimension. They are capable of measuring geographic topology from space down to centimeter resolution. They are being used in astronomy to measure stellar structures. They are used at the other end of the scale to measure stress in crystal structures, with intraatomic precision.