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?

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