 Interferometry makes use of the principle of superposition to combine waves in a way that will cause the result of their combination to have some meaningful property that is diagnostic of the original state of the waves . This works because when two waves with the same frequency combine , the resulting intensity pattern is determined by the phase difference between the two waves — waves that are in phase will undergo constructive interference while waves that are out of phase will undergo destructive interference . Waves which are not completely in phase nor completely out of phase will have an intermediate intensity pattern , which can be used to determine their relative phase difference . Most interferometers use light or some other form of electromagnetic wave . 
