In a Young’s double slit experiment using mono-chromatic light, the fringe pattern shifts by a certain distance on the screen when a mica sheet of refractive index 1.6 and thickness 964 micron (1 micron = 10-6 m) is introduced in the path of one of the Interfering waves. The mica sheet is then removed and the distance between the screen and the slits is doubled. �t is found that the distance between the successive maxima now is the same as the observed fringe-shift upon the introduction of the mica sheet. Calculate the wavelength of the monochromatic light used in the experiment.

Given, refractive index of mica,

Thickness of mica,


Let D be the distance between the slits and the screen, “d” be the distance between two slits and be the wavelength of light.


Now, the fringe width is given by


The path difference when the mica sheet is present is


Therefore, the number of fringe shifted,


And the shift in the fringes is given by the fringe width multiplied by the number of fringes shifted.



And for the second case, the fringe width is,



Here, there’s no shift present.


From the question,






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