Question

Why light comes back in same way when passing through centre and why it becomes parallel in concave mirror

Answer 1

When passing through the center the light comes back in the same way, because there is no angle between the light ray and the mirror at that point, or the angle is 90 degrees, so you can think of it as light getting reflected from a plane mirror, and it is true only for that point alone and that middle point is called the pole.

When light comes from the focus of the concave mirror it becomes parallel after reflection, because the concave mirror is made in that way, and we only study the properties of it.