First slide
In YDSE one of the slits is covered with a thin transparent sheet
Question

When one of the slits of Young's experiment is covered with a transparent sheet of thickness 4.8 mm, the central fringe shifts to a position originally occupied by the 30th bright fringe. What should be the thickness of the sheet if the central fringe has to shift to the position occupied by 
20th bright fringe?

Moderate
Solution

If shift is equivalent to n fringes then

n=(μ1)tλntt2t1=n2n1t2=n2n1×t

t2=2030×4.8=3.2mm

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