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A parallel beam of white light falls on a thin film whose refractive index is equal to 4//3. The angle of incidence i=53^@. What must be the minimum film thickness if the reflected light is to be coloured yellow most intensively? (tan53^@ =4//3) |
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Answer» `:. 4/3 = (sin 53^@)/(sin r) = (4//5)/(sin r)`  `:. Sin r =3/5` `:. r = 37^@` Refer FIGURE (a)  `Deltax_1` = between 2 and 1 = `2(AD)` ` = 2BD sec r ` `=2t sec r ` Their OPTICAL path `Delta x_1= 2mu t sec r. ` Refer figure (B)  `Deltax_2 = AC sin i = (2t tan r)sin i ` `:. (Delta x)_"net" = Deltax_1 - Deltax_2` ` = 2mutsecr = 2t (tanr)(sin i)` `= 2 xx 4/3 xx t xx 5/4 -2 xx t xx 3/4 xx 4/5 =32/15 t ` Phase difference between 1 and 2 is `pi`. `:.` For constructive interference, `32/15 t = lambda/2` or `t = (15lambda)/64 = (15 xx 0.6)/64` ` = 0.14 mu m `. |
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