Without a sample there is no phase difference for the two beams in detector 1 , thus yielding constructive interference : both have incurred wavelength + '' k '' phase shift due to two front - side reflections and one transmission through a glass plate .
Without a sample , there is no phase difference in the two beams in detector 1 , yielding constructive interference . 
 Both beams will have undergone a phase shift of wavelength + '' k '' due to two front - side reflections and one transmission through a glass plate .