Grade 1 · lesson 2 of 4

Reflect Across a Center Line

A vertical reflection keeps height and reverses horizontal direction.

About 15 minutes, plus any extended practice

Supplied materials

Paper and a pencil for a grid, a center line and dots. You can also describe where each dot goes.
Read this example: A dot is 2 grid squares left of a vertical mirror line. Where does its reflected partner go? Supplied candidates: (A) Place it 2 squares above the line (B) Place it 3 squares right of the line (C) Place it on the line (D) Place the matching dot 2 squares right of the center line, at the original dot’s height.
Try this question: A dot is 3 grid squares left of a vertical mirror line. Where does its reflected partner go? Supplied candidates: (A) Place it 3 squares above the line (B) Place it 4 squares right of the line (C) Place it on the line (D) Place the matching dot 3 squares right of the center line, at the original dot’s height.

Learn the idea

A vertical reflection puts matching dots on opposite sides of a mirror line. The dots stay at the same height and the same distance from the line. Count from the line, not from the edge of the page.

Reason through the supplied case: A vertical reflection keeps height and reverses horizontal direction. Equal distance to the mirror line places the partner 2 squares to the right. Count from the center line rather than from the page edge.

Count from the mirror line. Matching distance without matching height does not place a correct reflected partner; paper dots avoid any need for an actual mirror.

Find the vertical mirror line. Count how many grid squares the dot is from that line.
Explain why the partner goes two squares to the right of the line at the same height.
Try the question and all three practice questions before checking their answers.
Draw a vertical line and three dots on its left, then add each reflected partner on the right.
Check your work: Each pair has equal line distance and equal height on opposite sides.

Worked example

See the method in action

A dot is 2 grid squares left of a vertical mirror line. Where does its reflected partner go? Supplied candidates: (A) Place it 2 squares above the line (B) Place it 3 squares right of the line (C) Place it on the line (D) Place the matching dot 2 squares right of the center line, at the original dot’s height. Model decision: Place the matching dot 2 squares right of the center line, at the original dot’s height. A vertical reflection keeps height and reverses horizontal direction. Equal distance to the mirror line places the partner 2 squares to the right. Count from the center line rather than from the page edge.

Your turn

Check your reasoning

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Helpful tips

A vertical reflection keeps height and reverses horizontal direction.
Count from the mirror line.

See the answer and explanation

Place the matching dot 3 squares right of the center line, at the original dot’s height

A vertical reflection keeps height and reverses horizontal direction. Equal distance to the mirror line places the partner 3 squares to the right. Count from the center line rather than from the page edge.

Take it further

Try one more question: A dot is 9 grid squares left of a vertical mirror line. Where does its reflected partner go? Supplied candidates: (A) Place it 10 squares right of the line (B) Place it 9 squares above the line (C) Place it on the line (D) Place the matching dot 9 squares right of the center line, at the original dot’s height. Answer: Place the matching dot 9 squares right of the center line, at the original dot’s height. A vertical reflection keeps height and reverses horizontal direction. Equal distance to the mirror line places the partner 9 squares to the right. Count from the center line rather than from the page edge. Now make your own work: Draw a vertical line and three dots on its left, then add each reflected partner on the right. Check your work: Each pair has equal line distance and equal height on opposite sides.

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More practice: try new cases

Choose an answer for each problem. Explain your method on paper, then use the feedback to decide what to revisit.

1. A dot is 4 grid squares left of a vertical mirror line. Where does its reflected partner go?Place it 4 squares above the linePlace it 5 squares right of the linePlace the matching dot 4 squares right of the center line, at the original dot’s heightPlace it on the line

Need a hint?

A vertical reflection keeps height and reverses horizontal direction.
Count from the mirror line.

2. A dot is 5 grid squares left of a vertical mirror line. Where does its reflected partner go?Place it 5 squares above the linePlace it 6 squares right of the linePlace it on the linePlace the matching dot 5 squares right of the center line, at the original dot’s height

Need a hint?

A vertical reflection keeps height and reverses horizontal direction.
Count from the mirror line.

3. A dot is 6 grid squares left of a vertical mirror line. Where does its reflected partner go?Place the matching dot 6 squares right of the center line, at the original dot’s heightPlace it 6 squares above the linePlace it 7 squares right of the linePlace it on the line

Need a hint?

A vertical reflection keeps height and reverses horizontal direction.
Count from the mirror line.

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Check the subject sources

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