In Young's experiment transparent thin sheets are placed in front of the two slits, so that the central bright fringe stays at the same position. The thicknesses of the two sheets are t₁ and t₂ and their refractive indices are n₁ and n₂. In this case

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(3) (t₁)/(t₂)=(n₂-1)/(n₁-1). Given: sheets of thickness t₁, t₂ and refractive index n₁, n₂ in front of the two slits, with the central bright fringe still where it was. Idea: a sheet of thickness t and index n replaces t of air, so it lengthens the optical path of that arm by (n-1)t -- the light travels the same distance but at a slower speed. The central fringe sits where the two optical paths are equal, so it can only stay put if both arms were lengthened by the same amount. (n₁-1)t₁=(n₂-1)t₂. Rearranging, (t₁)/(t₂)=(n₂-1)/(n₁-1). So the thicker sheet has to be the one with the smaller (n-1).

In Young's experiment transparent thin sheets are placed in front of the two slits, so that the central bright fringe stays at the same position. The thicknesses of the two sheets are t₁ and t₂ and their refractive indices are n₁ and n₂. In this case

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Why the other options are wrong