If we make these tyro rays of light interfere at
then, when the apparatus is turned through 90°, the interference fringes should become shifted. The amount of this displacement may easily be calculated. If we denote by
the vibration frequency of the light-ray used in the experiment, then
is the corresponding wave-length. Thus, expressed in fractions of the interval between the fringes, the expected displacement becomes equal to
By causing the light to be reflected many times