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Find the thickness of a plate which will...

Find the thickness of a plate which will produce a change in optical path equal to half the wavelength `lamda` of the light passing through it normally. The refractive index of the plate is `mu`.

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Optical Path : If light travels a distance 'x' in a medium of refractive index `mu`, the equivalent path in vacuum, in would travel in the same time is `mu x`, which is called as optical path.
Change in optical path `= mu x - x = (mu - 1)x`
According to problem `(mu - 1)x = lambda`
Hence, `x = (lambda)/(mu - 1)` = thickness of plate.
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