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This is the question in my text :

A monochromatic light of wavelength $\mathrm{600 \times 10^{-9}m}$ is used in a Young's double slit experiment. One of the slit is covered by a transparent sheet of thickness $\mathrm{1.8 \times 10^{-5}m}$ made of a material of refractive index 1.6 . How many fringes will shift due to introduction of the sheet?

I don't know how to solve the above problem, but i am even not able to understand the solution given in the solution book. Author has written a statement:

One fringe shifts when the optical path changes by one wavelength.

i am not able to understand this statement also.

Can anyone please tell me how to solve this problem.

EDIT

i don't want the procedure but i want what does that statement mean which author has given.

i also want how to find optical path change.

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closed as off-topic by AccidentalFourierTransform, David Z Apr 27 '16 at 13:56

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