Practice portal › Wave Optics › Interference and Young's Double Slit Experiment
Asked in GSEB Board July 2017 · Slabs, media and modified set-ups
Given: a plate of refractive index μ = 1.5 in the path of one of the two interfering rays, with the central fringe still bright.
Idea: putting the plate in replaces a thickness t of air by the same thickness of glass, which lengthens that ray's optical path by (μ - 1)t.
The centre stays bright only if this extra path is a whole number of wavelengths, so (μ - 1)t = mλ with m = 1, 2, 3, …
The smallest thickness is the one with m = 1: t = λ/(μ-1).
t = λ/(1.5 - 1) = λ/(0.5).
The minimum thickness of the plate is 2λ.
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