In learning to derive the diffraction grating equation to form an interference pattern on a screen parallel to the diffraction grating, this kind of image is used extensively:
Then $AB = d$, $AC = n\lambda$ then $d\sin\theta = n\lambda$
This assumes the lines are parallel. However, in every derivation I've searched, it states this, but not if this is an approximation as my intuition states:
My possibly flawed understanding is that these 3 light rays shown in the picture would converge onto a single point and constructively interfere, creating a maxima, however this means that these light rays aren't parallel.
Is there something I'm missing? Are they parallel enough to ignore the small differences in angles, or are they actually parallel?