Manipal Entrance Test 2015 Physics Paper

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