When a thin film of thickness t is placed in the path of the light wave emerging out of S then increase in length of optical path will be
Monochromatic light (single wavelength) falls on two narrow slits S1and S2 which are very close together acts as two coherent sources, when waves coming from two coherent sources s1 and s2 superimposes on each other, an interference pattern is obtained on the screen.In YDSE alternate bright and dark bands obtained on the screen.these bands are called fringes.
When a film of thickness’t’ and refractive index 'm' is introduced in the path of one of the sources, then fringe shift occurs as the optical path difference changes.
Optical path difference changes
Optical path difference at
P = S2P - [S1P+ µt - t] = S2P - S1P - (µ - 1)t = y.d/D - (µ - 1)
⇒ nth fringe is shifted by Δy = D(µ-1)t/d = w/λ (µ-1)t
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