 The Mach – Zehnder interferometer is a more versatile instrument than the Michelson interferometer . Each of the well separated light paths is traversed only once , and the fringes can be adjusted so that they are localized in any desired plane . Typically , the fringes would be adjusted to lie in the same plane as the test object , so that fringes and test object can be photographed together . If it is decided to produce fringes in white light , then , since white light has a limited coherence length , on the order of micrometers , great care must be taken to equalize the optical paths or no fringes will be visible . As illustrated in Fig . 6 , a compensating cell would be placed in the path of the reference beam to match the test cell . Note also the precise orientation of the beam splitters . The reflecting surfaces of the beam splitters would be oriented so that the test and reference beams pass through an equal amount of glass . In this orientation , the test and reference beams each experience two front @-@ surface reflections , resulting in the same number of phase inversions . The result is that light traveling an equal optical path length in the test and reference beams produces a white light fringe of constructive interference . 
