Translate, reflect & rotate shapes
Theory
A translation slides a shape, a reflection flips it over a mirror line, and a rotation turns it about a centre. All three keep the shape and size the same, so the image is congruent to the original. This Year 6 maths page shows how to carry out and describe each transformation on the number plane, find where points move to, and combine two transformations.
A translation slides every point the same distance in the same direction. Describe it as "\(4\) units left and \(5\) units down".
A reflection flips a shape over a mirror line. Each image point is the same distance from the line, on the other side. Reflecting in the \(x\)-axis changes the sign of \(y\); reflecting in the \(y\)-axis changes the sign of \(x\).
A rotation turns a shape about a centre of rotation by an angle: a quarter turn is \(90^\circ\), a half turn \(180^\circ\), clockwise or anticlockwise.
The image is the shape after the move. It is congruent (same shape and size) to the original. Only a reflection makes it face the opposite way.
Transformations on the number plane:
| Move | Example |
|---|---|
| Translation | \(4\) left, \(5\) down |
| Reflect in \(x\)-axis | \((5, 2) \to (5, -2)\) |
| Reflect in \(y\)-axis | \((2, 4) \to (-2, 4)\) |
| Half turn about \((0, 0)\) | \((-4, 1) \to (4, -1)\) |
Describing a transformation
- Same way round, just moved? Translation: give the distance across and up/down.
- Flipped? Reflection: name the mirror line.
- Turned? Rotation: give the centre, the angle and the direction.
Finding an image point
- Translation: add or subtract the moves from \(x\) and \(y\).
- Reflection: count the distance to the mirror line and go the same distance past it.
- Rotation: turn the shape (tracing paper helps) and read the new corners.
The corner \((5, 5)\) moves to \((1, 0)\).
| \(5 - 4\) | \(=\) | \(1\) |
| \(5 - 5\) | \(=\) | \(0\) |
4 units left and 5 units down.
It is \(2\) above the mirror line, so the image is \(2\) below: \((5, -2)\).
(i) Through how many degrees does the triangle turn?
(ii) Where does the corner \((4, 3)\) go?
(i) \(90^\circ\) anticlockwise
(ii) \((-3, 4)\)
A half turn sends it to the opposite side of the origin: \((4, -1)\).
A translation.
| \(4 - 3\) | \(=\) | \(1\) |
\((2, 4) \to (-2, 4) \to (-2, 1)\).
(i) How long are the sides of the final image?
(ii) What is its area?
| \(9 \times 4\) | \(=\) | \(36\) |
(i) \(9\) cm and \(4\) cm
(ii) \(36\,\text{cm}^2\), the same as before
Reflecting in the \(y\)-axis changes the sign of the \(x\)-coordinate, not the \(y\)-coordinate, so the image is \((-3, 2)\). The point \((3, -2)\) is the reflection in the \(x\)-axis.
Common pitfalls
Frequently asked questions
What is a translation?
A slide: every point moves the same distance in the same direction.
What happens to (5, 2) when it is reflected in the x-axis?
It becomes \((5, -2)\).
How many degrees is a quarter turn?
\(90^\circ\).
Does a transformation change the size of a shape?
No. Translations, reflections and rotations keep the shape and size.
How can I tell a reflection from a rotation?
A reflection makes a mirror image that faces the other way; a rotation does not.