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Visible light of variable wavelength is incident normally on a thin sheet of plastic in air. The reflected light has a minima only for `lamba=512` nm and `lambda=640` nm in the visible spectrem. What is the minimum thickness (in mum) of the film `(mu=1.28)` ? |
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Answer» Correct Answer - `0001` Reflected ray from upper surface would would shift by `lambda//2` only while reflected from lower surface would not have any shift. `2mut=n_(1)lambda_(1)=n_(2) lambda_(2) Rightarrow (n_(1)=n_(2)+1)`as there is no minima in between these two wavelengths `(n+1)(512)=(n)(640)` `n_(2)(640-512)=512` `n_(2)=4` So `2xx1.28t=(4)(640)` `t=(4xx640)/(2xx1.28)=1000` nm=1mum |
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