The meeting of two wave systems resulting in increased amplitude (constructive interference) if they are in phase, i.e., crest to crest, and decreased amplitude (destructive interference) if they are out of phase, i.e., crest to trough. In polarized-light microscopy (PLM), phase differences are generated when white light passes through an anisotropic (i.e., doubly refracting) crystal or crystal fragment, these differences being determined primarily by birefringence, light wavelength, and crystal thickness. Waves of light in different parts of the visible spectrum interfere both constructively and destructively when resolved in the microscope analyzer to give an interference color. Since anisotropic minerals have a range of birefringence, interference colors are useful as an aid to their identification.