Question

What is the thinnest soap film (excluding the case of zero thickness) that appears black when illuminated with light with a wavelength of 515 nm ? The index of refraction of the film is 1.32, and there is air on both sides of the film.

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I know that it would appear dark when there is destructive reflection, the equation for which is:
2t = (m + .5)lambda (where t is thickness, and no phase shift)
but I don't know how to incorporate the index of refraction n for air etc.
Any help would be appreciated, thanks

Answer 1

lambda is speedlight/freq

lambda in soap= (c/1.5)(1/c/515nm)=515nm/1.5