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Concept

Reflection of Geometric Objects

A reflection is a transformation in which every point of a figure is reflected in a line. The line in which the points are reflected is called the line of reflection and acts like a mirror.
Triangle being reflected across a movable line
More precisely, a reflection across a line maps every point in the plane into its image such that one of the following statements is satisfied.
  • If is on the line then and are the same point.
  • If is not on the line then is the perpendicular bisector of
Segment AA' intersects line ell perpendicularly, and line ell bisects segment AA'. Points B and B' coincide.

Like rotations and translations, reflections are also rigid motions because they preserve the side lengths and angle measures. However, reflections change the orientation of the preimage.