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A thin plastic of refractive index 1.6 i...

A thin plastic of refractive index 1.6 is used to cover one of the slits of a double slit arrangement. The central point omn the screen is now occupied by what would have been the 7th bright fringe before the plastic was used. If the wavelength of light is 600 nm, what is the thickness ( in `mu`m ) of the plastic?

A

7

B

4

C

8

D

6

Text Solution

Verified by Experts

Fringe width of the seven fringes =`7 xx (D)/(2d) lambda`
Again displacement of the central bright fringe for the plastic sheet of thickness t =` (D)/(2d) (mu -1)`t
According to the question
`7(D)/(2d) lambda=(D)/(2d)(mu-1)`t
[ Here `mu` =1.6 , `lambda`=600nm =0.6 `mu`m]
`:. t =(7lambda)/(mu-1)=(7xx0.6)/(1.6-1)=7 mu`m
The option A is correct.
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