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#### Q. A thin film of soap solution (n = 1.4) lies on the top of a glass plate (n = 1.5). When visible light is incident almost normal to the plate, two adjacent reflection maxima are observed at two wavelength 400 and 630 nm. The minimum thickness of the soap solution is?

For reflection at the air-soap solution interface, the phase difference is π
For reflection at the interface of soap solution to glass also, there will be a phase difference of π.
∴ The condition for the maximum intensity
=2μt=nλ
For n, nλ1​=(n−1)λ2​
n×420=(n−1)630
∴n(630−420)=630
⇒n(210)=630
⇒n=210630​
⇒n=3
This is the maximum order, where they coincide
2×1.4×t=3×420
⇒t=2×1.403×420​=450n

#### Q. Maximum diffraction takes place in a given alit for?

Greater is the wavelength of wave higher will be its degree of diffraction.