What if you were given the coordinates of a quadrilateral and you were asked to reflect that quadrilateral over the axis? What would its new coordinates be? After completing this Concept, you'll be able to reflect a figure like this one in the coordinate plane.

### Watch This

Transformation: Reflection CK-12

### Guidance

*A* *transformation**is an operation that moves, flips, or otherwise changes a figure to create a new figure. A* *rigid transformation**(also known as an* **isometry** *or* **congruence transformation***) is a transformation that does not change the size or shape of a figure.*

*The rigid transformations are Transformation: Translation, reflections (discussed here), and Transformation: Rotation. The new figure created by a transformation is called the* **image***. The original figure is called the* **preimage***. If the preimage is , then the image would be , said “a prime.” If there is an image of , that would be labeled , said “a double prime.”*

A **reflection** is a transformation that turns a figure into its mirror image by flipping it over a line. The **line of reflection** is the line that a figure is reflected over. If a point is on the line of reflection then the image is the same as the preimage. Images are always congruent to preimages.

While you can reflect over any line, some common lines of reflection have rules that are worth memorizing:

**Reflection over the axis:**

**Reflection over the axis:**

**Reflection over :**

**Reflection over :**

#### Example A

Reflect over the axis. Find the coordinates of the image.

will be the same distance away from the axis as , but on the other side. Hence, their -coordinates will be opposite.

#### Example B

Reflect the letter over the axis.

When reflecting the letter over the axis, the coordinates will be the same distance away from the axis, but on the other side of the axis. Hence, their -coordinates will be opposite.

#### Example C

Reflect the triangle with vertices and over the line . Find the coordinates of , , and .

The image’s vertices are the same distance away from as those of the preimage.

Transformation: Reflection CK-12

### Guided Practice

1. Reflect the line segment with endpoints and over the line .

2. A triangle and its reflection, are below. What is the line of reflection?

3. Reflect square over the line .

4. Reflect the trapezoid over the line .

**Answers:**

1. is on the line of reflection, which means has the same coordinates. is the same distance away from , but on the other side.

2. Looking at the graph, we see that the corresponding parts of the preimage and image intersect when . Therefore, this is the line of reflection.

If the image does not intersect the preimage, find the midpoint between the preimage point and its image. This point is on the line of reflection.

3. The purple line is . Fold the graph on the line of reflection.

4. The purple line is . You can reflect the trapezoid over this line.

### Practice

- If (5, 3) is reflected over the axis, what is the image?
- If (5, 3) is reflected over the axis, what is the image?
- If (5, 3) is reflected over , what is the image?
- If (5, 3) is reflected over , what is the image?
- Plot the four images. What shape do they make? Be specific.
- Which letter is a reflection over a vertical line of the letter ?
- Which letter is a reflection over a horizontal line of the letter ?

Reflect each shape over the given line.

- axis
- axis
- axis
- axis

Find the line of reflection the blue triangle (preimage) and the red triangle (image).