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