Reflecting in a Line
Part of the Transformations & Constructions section of Coddy's Geometry journey. Lesson 6 of 61.
A reflection flips a shape over a mirror line. Each point goes straight across the mirror, at right angles to it, and lands the same distance away on the other side. A point on the mirror stays where it is.
Here the mirror is the level line y = 2. P(1, 5) is 3 above it, so P' is 3 below it, at (1, -1). R(1, 3) is 1 above, so R'(1, 1) is 1 below. The image has the same size and shape, but it is flipped: read P, Q, R round the triangle one way and P', Q', R' go round the other way. That flip tells a reflection apart from a slide or a turn.
On the board, the move is Reflect. Set the mirror: x = a for an upright mirror, y = b for a level one, and type the number. Then press Apply.
Challenge
Find the mirror that flips triangle ABC onto the dashed target: set the mirror line, press Apply, then press Check.
Try it yourself
Move the shape onto its target
Test yourselfThis lesson includes a short quiz. Start the lesson to answer it and track your progress.
All lessons in Transformations & Constructions
7Drawing on Squared Paper
Lengths by PythagorasPerpendicular StepsSquares by AreaRight Triangles by AreaMidpoints on the GridRecap: Squared Paper10Perpendiculars and Parallels
Perpendicular From a PointPerpendicular at a PointParallel Through a PointTools You Have EarnedRecap: Parallels5Dilations
Scaling From a CenterShrinking a ShapeDilating About a PointNegative Scale FactorsRecap: Dilations8Compass and Straightedge
Circles Keep DistancesWhy the Sides MatchAngles You BuiltA 60° AngleRecap: Equilateral Triangle