Question

Imagine a soap film (air-soapy water-air) of 1 l,m thickness, and refractive index of 1.5 with white light normal to the film. Which wavelengths in the optical range (400 nm to 700 nm) will appear brightest?

Answer 1

positive interference will occur when the thickness is 360 degrees. One gets 180 phase change on the reflection, so thickness is 1/4 wavlength.

I do not understand your thickness above, but you want thickness to be 1/4 *lambda