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A thin transparent film of thickness 3000 `Å` and refractive index 1.5 ud deposited on a sheet made of a metal. Assuming normal incidence of light and also that the film is a plane parallel one, what will be the color of a pot made from this sheet when observed in white light? |
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Answer» Coditiions for maximum and minimum intensity in reflected light, in case of thin film interference, are Maxima: `2 mu t = (2n 1) lambda//2, n = 1, 2,…` Minima: `2 mu t = n lambda, n = 0, 1, 2, 3,…` Given: `mu = 1.5 , t = 3000 Å = 3000 xx 10^(10) m` (normal incidence) Maxima: `2 mu t = (2 n - 1) lambda//2` `:. 2 xx 1.5 xx (3000 xx 10^(-10)) = (2n - 1) lambda//2` or `lambda = (4 xx 1.5 xx 3000 xx 10^(-10))/(2n -1)` `:. lambda =(18000)/(2n - 1) xx 10^(-10) m` For `n = 1,lambda = 18000 Å` `n = 2, lambda = 6000 Å` `n = 3, lambda = 3600 Å` and so on Only `lambda = 6000 Å` for `n = 2` lies in the visible range. Intensity will be minimum for `lambda = 4500 Å` which falls in the violet-blue pat of the visible spectrum. When observed in white light, the red -orange part will be strongly reflected and the violet-blue part will be quite reduced in intensity. Hence, Colour of the pot will appear to be orange-red. |
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