what is the way you find the thickness of a detergent film?

Answer 1

You can count fringes when illuminating the film with a monochromatic light source. This will give you relative thicknesses across the film. You can make an assumption that at the top where there are NO fringes and the dark part keeps stretching that the thickness is less than half the wavelength of light you are using and at the first bright fringe it is about the wavelength over 2 thick.

If you use a sodium lamp for your light source, take care to use a low pressure sodium lamp. High pressure lamps are nothing near monochromatic

Answer 2

Good question, thanks for the 2 points!