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White light of spectral range 4000 - 7000 Å is incident normally on a soap film of uniform thickness 0.0004 mm and mu = 1.3. The wvaelengths of the light which are strongly reflected are : |
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Answer» 4000 and 6000 `Å` Thickness of film, `t = 0.0004 MM` `= 4 xx 10^(-5) cm` and `mu = 1.3` For normal incidence` i = r = 0^(@)` Using condition for brightness `2mu t cos r = (2n + 1)(LAMBDA)/(2)` `2 xx 1.3 xx 4 xx 10^(-5) = (2n + 1).(lambda)/(2)` `therefore lambda = (20,800)/(2n + 1) Å` When `n = 0, lambda = 20,800 Å` `n = 1, lambda = (20,800)/(3) Å = 6933 Å` `n = 2 , lambda = (20,800)/(5) Å = 4160 Å` `n = 3 , lambda = (20,800)/(7) Å = 2972 Å` So required wavelengths are `6933 Å` and `4160 Å`. |
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