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I'm trying to figure out how thin films work when they're not flat. In a vertical soap film, for example, the film is thicker at the bottom, being pulled down by gravity.

soap film

My question is whether interference occurs in case B, or if it's only when the light rays are coincident like in A:

interference diagram

If B is possible, then where would the image show up: in the direction of the blue ray or the red ray? And if that's not the way interference works, how is it possible that thin film interference occurs on the soap film? Thanks so much!

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Yes, interference can occur for non-coincident beams, provided that their electric fields are not orthogonal. Just treat the $x$, $y$, and $z$ components of each field at each point separately, and see how they add.

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  • $\begingroup$ oh okay thank you! and to follow up, if the observer were at the point where the red and blue lines intersect in diagram b, then where would the image appear? $\endgroup$
    – michael
    Commented Nov 12, 2018 at 19:02

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