 Typically ( see Fig . 1 , the well @-@ known Michelson configuration ) a single incoming beam of coherent light will be split into two identical beams by a beam splitter ( a partially reflecting mirror ) . Each of these beams travels a different route , called a path , and they are recombined before arriving at a detector . The path difference , the difference in the distance traveled by each beam , creates a phase difference between them . It is this introduced phase difference that creates the interference pattern between the initially identical waves . If a single beam has been split along two paths , then the phase difference is diagnostic of anything that changes the phase along the paths . This could be a physical change in the path length itself or a change in the refractive index along the path . 
