why does a parting in our hair on the left appear to be a parting on the right hand when seen in a mirror

Dear Student!

It happens because of lateral inversion. A beam of parallel rays is just reflected back by a plane mirror. Thus, rays in left initially get into the right of reflected beam and vice-verca. Similar phenomenon occurs when you stand in front of a mirror. Your right is actually left of the image. This is called lateral inversion.