# Definition:Reflection (Geometry)/Plane

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## Definition

A **reflection** $\phi_{AB}$ in the plane is an isometry on the Euclidean Space $\Gamma = \R^2$ as follows.

Let $AB$ be a distinguished straight line in $\Gamma$, which has the property that:

- $\forall P \in AB: \map {\phi_{AB} } P = P$

That is, every point on $AB$ maps to itself.

Let $P \in \Gamma$ such that $P \notin AB$.

Let a straight line be constructed from $P$ to $O$ on $AB$ such that $OP$ is perpendicular to $AB$.

Let $PO$ be produced to $P'$ such that $OP = OP'$.

Then:

- $\map {\phi_{AB} } P = P'$

Thus $\phi_{AB}$ is a **reflection (in the plane) in (the axis of reflection) $AB$**.

### Axis of Reflection

Let $\phi_{AB}$ be a reflection in the plane in the straight line $AB$.

Then $AB$ is known as the **axis of reflection**.

## Sources

- 2014: Christopher Clapham and James Nicholson:
*The Concise Oxford Dictionary of Mathematics*(5th ed.) ... (previous) ... (next): Entry:**reflection**(of the plane)