Reflection (In x-axis, y-axis, x = a, y = a and the origin; Invariant Points)
297. Mirror Image on Y-Axis · Flip the x, keep the y
The segment PP’ is always perpendicular to the y-axis and bisected by it.
Reflecting in the y-axis sends (x, y) → (−x, y): the y-coordinate stays and the x-coordinate changes sign. The y-axis is the perpendicular bisector of the segment joining a point to its image.