Reflection (In x-axis, y-axis, x = a, y = a and the origin; Invariant Points)
295. The Mirror’s Rule · Reflection as a perpendicular bisector
The mirror line is always the perpendicular bisector of segment PP′.
Under a reflection, the mirror line is the perpendicular bisector of the segment joining each point to its image. So the mirror lies exactly halfway between P and P′ and at right angles to PP′.