What is the minimum plastic film thickness required for constructive interference?

GGG: What is the minimum plastic film thickness required for constructive interference?
A thin plastic film with an index of refraction of 1.3 is put on the surface of a car window (n=1.5) to increase the window reflectivity. What is the minimum plastic film thickness required if light of wavelength 550 nm in air is reflected from the two sides of the film and interferes constructively?

Answers and Views:

Answer by DeviantArt
since 1 < 1.3 < 1.5, condition 180deg phase difference. So condition for constructive interference and destructive interference is reversed. Minimum thickness will be measured using formula t = λ/2n t= 211.53nm