Why light comes out in spherical form after passing through a slit (diffraction)? 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?
 

 A: *

*When wave meets the slit, slit becomes secondary sources of wave (secondary wavelets). Since different directions are equal, the secondary wave must be spherical.

*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
A: First picture is a wonderful example, how diffraction of electromagnetic radiation does not work. Indeed behind a slit for water waves one could see spherical waves distributed all over the area behind a wall. For light this is impossible, the diffraction happens only in a small angle. For light there are intensive distributions in a small area left and right from the end of the geometric shadow. For diffraction experiments with electrons there are not fringes inside the geometrical shadow at all.
More important is to see the difference between the pattern of water waves and fringes from light. What Young was not able to do, we are doing today easily. Youngs sketch of water waves is a snapshot of your first picture. The difference is, that the intensity distribution of water waves is moving, the intensity distribution of light diffraction is stationary. That is, why Youngs conclusion from water waves to light  as a wave does not satisfies me.
To your second question. Diffraction happens behind every edge, bringing two edges close enough together one get twice the pattern from one edge. Now, moving two slits together, one get different intensity distributions behind the slits. In the center of the intensity distribution one can produce overlapping areas, but on both ends one will get the fringes like from single edges.
