Monochromatic light (vacuum = 661 nm) shines on a soap film (n = 1.33) that has air on either side of it. The light strikes the film perpendicularly. What is the minimum thickness of the film for which constructive interference causes it to look bright in reflected light?

Answer 1

The reflective path has to be a multiple of whole wavelengths. Because the back side gives a half wave length shift, that means the thickness has to be 1/4 wavelength (or half wavelength both directions).
1/4*661nm=minimum thickness

Answer 2

For constructive interference, however, wouldnt you use 2t = m +1/2 wavelength. For this question m being 0 as atmospheric is equal to vacuum. Which gives 2t = 1/2 wavelength/n, simplifying to t = wavelength/4n