Light of wavelength 6000Å is incident on a thin glass plate of refractive index 1.5 such that angle of refraction into the plate is `60^(@)`. Calculate the smallest thickness of plate which will make it appear dark by reflection.
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Here, `lambda = 6000 Å = 6 xx 10^(-7)m`, `mu = 1.5, r = 60^(@), t = ?` For the film to appear dark in the reflected system, `2mut cos r = n lambda` For smallest thickness of film, `n = 1` `:. 2 mut cos r = lambda` ,brgt `t = (lambda)/(2 mu cos r) = (6 xx 10^(-7))/(2 xx 1.5 cos 60^(@)) = 4 xx 10^(-7)m`
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