Can interference occur between two waves that are parallel but separated by a small distance?

Question

This is a image of diffraction in crystal. My doubt is how the parallel waves coming out interfere if they are seperate?

Answer 1

In answering your question, a lot could be said about the art of mathematical modeling, but, the short answer is: They don't.

But, the rays in the scheme are only an approximation, and one that fails at the atomic scale - a beam of light, no matter how laser like or faint, is never exactly a 1-D mathematical line, it spreads sideways. That's why they can interfere.