Consider the experimental setup for Young's Double Slit Experiment:


[![enter image description here][1]][1]


  [1]: https://i.sstatic.net/IbPnY.png


Here there is an angle between the directions of propagation of waves ,i.e., $\angle S_1PS_2$

My question is:

Why is this angle never taken into account while analysing this experiment or the diffraction experiments?

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Source of the image: https://www.embibe.com/study/effect-of-placing-thin-transparent-film-in-ydse-in-front-of-slits-concept