[Physics] Why light comes out in spherical form after passing through a slit (diffraction)

Question

I want to know why light comes out in spherical form when it passes through the slit?
I know this is due to diffraction but how?

And my second question, if there is no difference between diffraction and interference, then why diffraction and interference patterns are different?

Answer 1

1. When wave meets the slit, slit becomes secondary sources of wave (secondary wavelets). Since different directions are equal, the secondary wave must be spherical.
2. For the second question, please follow what Feynman said (in Feynman Lectures on Physics):

No one has ever been able to define the difference between interference and diffraction satisfactorily. It is just a quest of usage, and there is no specific, important physical difference between them. The best we can do is, roughly speaking, is to say that when there are only a few sources, say two interference sources, then the result is usually called interference, but if there is a large number of them, it seems that the word diffraction is more often used.

From his answer, you can understand why diffraction and interference patterns are different