Transformations in the Plane
You are playing pool and the red ball is hidden behind the cue ball's straight path. The only shot is a bank shot off the top cushion. Where exactly on the cushion should you aim? A drone that changes position, a video game character flipped in a mirror, a cell tower moved because of interference: all of these are transformations of the plane. Each one can be described with a precise vector operation, and all of them are rigid motions: the figure moves, but its size and shape do not change. At the end of this page you will solve the pool shot with a single reflection.
Translations
A translation slides every point of a figure the same distance in the same direction. A vector tells you how far and which way.
Some problems write a vector with parentheses, like $(-5, 4)$. It is the same thing as $\langle -5, 4 \rangle$. Words become signs: "3 units to the left and 2 units up" is $\langle -3, 2 \rangle$.
A delivery drone is at position $P(3, -2)$, expressed in miles with respect to its base. The controller sends it an instruction to make a rectilinear displacement…
A delivery drone is at position $P(3, -2)$, expressed in miles with respect to its base. The controller sends it an instruction to make a rectilinear displacement defined by the translation vector $\vec{v} = (-5, 4)$. Determine the coordinates of the drone's new position.
A laser cutting machine moves in a straight line over a metal plate from the initial point $A(-1, 5)$ to the final point $B(4, 2)$. Calculate the translation vector that…
A laser cutting machine moves in a straight line over a metal plate from the initial point $A(-1, 5)$ to the final point $B(4, 2)$. Calculate the translation vector that exactly describes this movement.
A crane moves a rectilinear beam horizontally and vertically, whose initial ends are $A(1, 1)$ and $B(5, 3)$. The movement is carried out by a translation defined by the…
A crane moves a rectilinear beam horizontally and vertically, whose initial ends are $A(1, 1)$ and $B(5, 3)$. The movement is carried out by a translation defined by the vector $\vec{t} = (0, -4)$. Calculate the coordinates of the new ends $A'$ and $B'$ of the beam.
Reflection through a point
A point reflection flips a figure through a center, like looking at it "through" a pin. Each point goes to the other side of the center at the same distance.
In the design of a symmetrical garden, a fountain is to be placed at point $F'$ which is the reflection of a statue located at $E(4, 7)$ with respect to a central post…
In the design of a symmetrical garden, a fountain is to be placed at point $F'$ which is the reflection of a statue located at $E(4, 7)$ with respect to a central post located at the origin of coordinates $O(0, 0)$. Find the coordinates where the fountain should be installed.
Two communications satellites, $S_1$ and $S_2$, orbit symmetrically with respect to a central space station. If $S_1$ is at position $(8, -3)$ and the space station is…
Two communications satellites, $S_1$ and $S_2$, orbit symmetrically with respect to a central space station. If $S_1$ is at position $(8, -3)$ and the space station is located at $(2, 1)$, calculate the coordinates of the position of satellite $S_2$.
Reflection across a line
A mirror is a line. The reflection of $P$ across the line $r$ is the point $P'$ on the other side of the mirror, at the same distance, along the perpendicular.
Easy mirrors
For the axes and a few special lines you can write the image directly.
| Mirror | $(x, y) \to$ |
|---|---|
| $x$-axis ($y = 0$) | $(x, -y)$ |
| $y$-axis ($x = 0$) | $(-x, y)$ |
| $y = x$ | $(y, x)$ |
| $y = -x$ | $(-y, -x)$ |
| vertical line $x = h$ | $(2h - x,\ y)$ |
| horizontal line $y = k$ | $(x,\ 2k - y)$ |
For example, reflecting $V(5, -2)$ across $y = x$ just swaps the coordinates: $V' = (-2, 5)$. Reflecting across $x = 6$ gives $(12 - x, y)$.
Any mirror: perpendicular, intersect, midpoint
An architect designs a bridge whose arch has a critical anchor point at $A(-3, 5)$. To maintain visual symmetry in a 3D rendering with respect to the water surface,…
An architect designs a bridge whose arch has a critical anchor point at $A(-3, 5)$. To maintain visual symmetry in a 3D rendering with respect to the water surface, which coincides with the x-axis ($y=0$), they need to locate the virtual reflection of this point underwater. Determine the coordinates of this reflected point.
In a graphic design program, a polygon has been drawn with a key vertex at point $V(5, -2)$. A reflection is applied with respect to the bisector of the first and third…
In a graphic design program, a polygon has been drawn with a key vertex at point $V(5, -2)$. A reflection is applied with respect to the bisector of the first and third quadrants (line $y=x$). Determine analytically the coordinates of the reflected vertex $V'$.
A laser beam is emitted from a projector located at point $P(-2, 4)$. In front of it is a flat mirror placed exactly on the line with equation $r \equiv x - 2y + 5 = 0$.…
A laser beam is emitted from a projector located at point $P(-2, 4)$. In front of it is a flat mirror placed exactly on the line with equation $r \equiv x - 2y + 5 = 0$. Calculate the coordinates of the virtual point $P'$ (reflection of $P$ in the mirror) from which the reflected light appears to come.
The mirror between two points
Sometimes you know the point and its image and need the mirror. A reflection that sends $A$ to $B$ must have a mirror that cuts $\overline{AB}$ in half at a right angle.
Two forest watchtowers are located at points $T_1(-3, 1)$ and $T_2(5, 5)$. A rectilinear firebreak is to be drawn to act as an axis of symmetry between both towers, so…
Two forest watchtowers are located at points $T_1(-3, 1)$ and $T_2(5, 5)$. A rectilinear firebreak is to be drawn to act as an axis of symmetry between both towers, so that any point on the firebreak is equidistant from $T_1$ and $T_2$. Calculate the general equation of the line that defines this firebreak.
Shortest path with a bounce
Pool bank shots, laser beams and pipes with a stop on the way all share one idea: reflect one endpoint, then draw a straight line.
Answer: $A'B = \sqrt{5^2 + 5^2} = 5\sqrt{2} \approx 7.07$. Also $AP = 4\sqrt 2$ and $PB = \sqrt 2$, which add up to $5\sqrt 2$. ✓
Two towns, $A(1, 5)$ and $B(7, 2)$, need to connect to a main water pipe that runs in a straight line along the x-axis ($y=0$). A pumping station $P$ is to be built on…
Two towns, $A(1, 5)$ and $B(7, 2)$, need to connect to a main water pipe that runs in a straight line along the x-axis ($y=0$). A pumping station $P$ is to be built on the main pipe so that the total length of the secondary pipes ($AP + PB$) is minimized. Using the concept of reflection, calculate the exact coordinates of point $P$ and the minimum total length of the pipe.
On a pool table, a cue ball is located at point $B(2, 3)$ and a red ball at $R(8, 6)$. A player wishes to hit the red ball by making the cue ball bounce first off the…
On a pool table, a cue ball is located at point $B(2, 3)$ and a red ball at $R(8, 6)$. A player wishes to hit the red ball by making the cue ball bounce first off the top cushion of the table, which lies on the line $y = 10$. Assuming that the angle of incidence is equal to the angle of reflection, calculate the coordinates of the exact point on the cushion where the cue ball must hit to achieve their goal.
Translating equations of curves
To translate a whole curve you do not move infinitely many points. You rewrite the equation.
The path of a conveyor belt follows the line with equation $y = 2x - 1$. Due to space requirements in the factory, the entire belt is translated $3$ units to the left…
The path of a conveyor belt follows the line with equation $y = 2x - 1$. Due to space requirements in the factory, the entire belt is translated $3$ units to the left and $2$ units upward. Write the general equation of the new line that defines the translated path.
A cell tower provides coverage in a circular area whose Cartesian equation is $x^2 + y^2 - 4x + 6y - 12 = 0$. Due to interference problems with a nearby building, the…
A cell tower provides coverage in a circular area whose Cartesian equation is $x^2 + y^2 - 4x + 6y - 12 = 0$. Due to interference problems with a nearby building, the company decides to translate the tower (the center of the circle) according to the vector $\vec{v} = (-3, 5)$. Determine the general equation of the new coverage area.
The reflector of a car headlight follows the profile of the parabola $y = x^2 - 6x + 11$. To facilitate the programming of the CNC machining tool, the engineer needs to…
The reflector of a car headlight follows the profile of the parabola $y = x^2 - 6x + 11$. To facilitate the programming of the CNC machining tool, the engineer needs to apply a translation to the parabola so that its vertex coincides exactly with the origin of coordinates. Determine the necessary translation vector and the new equation of the parabola.
Moving a conic to the origin
Conics are easier to handle when they are centered at the origin. To get there, find the center and translate by the opposite vector.
The orbit of a comet is projected onto an astronomical plane according to the equation of the ellipse $4x^2 + 9y^2 - 24x + 36y + 36 = 0$. By means of a translation of…
The orbit of a comet is projected onto an astronomical plane according to the equation of the ellipse $4x^2 + 9y^2 - 24x + 36y + 36 = 0$. By means of a translation of axes, it is possible to express this equation in its reduced form centered at the origin. Calculate the translation vector that moves the geometric center of the conic to the origin of coordinates and write the equation of the ellipse after this translation.
Reflecting lines and polygons
To reflect a figure made of points and segments, reflect the key points and reconnect them. Reflections keep lengths, angles and area.
Reflecting a line through a point
When you reflect through a point $C(a, b)$, replace $x$ with $2a - x$ and $y$ with $2b - y$. The image of a line is a parallel line.
Reflecting a line across another line
Answer: $(2t - 1) - 2(t + 1) + 3 = 0$. ✓
A railroad track runs following the path of the line $r \equiv 3x + 4y - 12 = 0$. An urban planner wants to lay out a new avenue $s$ that is perfectly symmetrical to the…
A railroad track runs following the path of the line $r \equiv 3x + 4y - 12 = 0$. An urban planner wants to lay out a new avenue $s$ that is perfectly symmetrical to the railroad track with respect to a central monument located at $C(1, -1)$. Find the general equation of the avenue $s$.
An agricultural plot is shaped like a triangle with vertices at $A(0, 0)$, $B(4, 0)$ and $C(2, 5)$. The city council acquires the plot and, in exchange, gives the owner…
An agricultural plot is shaped like a triangle with vertices at $A(0, 0)$, $B(4, 0)$ and $C(2, 5)$. The city council acquires the plot and, in exchange, gives the owner another plot obtained by applying a reflection in the vertical line $x = 6$ to the original triangle. Calculate the vertices of the new plot and prove analytically that the area of the plot has not changed.
A ray of light travels along the line $r \equiv 2x - y + 4 = 0$ and hits a flat mirror located on the line $s \equiv x + y - 1 = 0$. Determine the general equation of…
A ray of light travels along the line $r \equiv 2x - y + 4 = 0$ and hits a flat mirror located on the line $s \equiv x + y - 1 = 0$. Determine the general equation of the line $r'$ containing the reflected ray, knowing that this line is the reflection of $r$ in $s$.
Reflecting a circle
A circle is defined by its center and radius. A reflection changes only the center. The radius stays the same, since distances are preserved.
The outline of a circular gear has the equation $x^2 + y^2 - 2x - 4y - 4 = 0$. This gear is reflected in an inclined mirror represented by the line…
The outline of a circular gear has the equation $x^2 + y^2 - 2x - 4y - 4 = 0$. This gear is reflected in an inclined mirror represented by the line $r \equiv 3x - 4y + 10 = 0$. Determine the general equation of the circle that represents the reflected image of the gear.
Composing transformations
To compose two transformations, apply one and then apply the second to the result.
In the picture, the solid path is "translate, then reflect" and the dashed path is "reflect, then translate". They end at different points.
Answer: Reflect first: $2C - A = (0, 1)$. Translate: $(0 + 3,\ 1 - 4) = (3, -3)$. Since $(3, -3) \ne (-3, 5)$, the order matters.
A fishing boat makes two consecutive movements on the high seas. First it translates according to the vector $\vec{u} = (2, 6)$ and then according to the vector…
A fishing boat makes two consecutive movements on the high seas. First it translates according to the vector $\vec{u} = (2, 6)$ and then according to the vector $\vec{v} = (3, -1)$. If it departed from a port located at coordinates $P(-4, -5)$, determine its final position after both displacements.
In a video game animation, the geometric center of a character located at $A(1, 3)$ undergoes a translation by vector $\vec{t} = (4, -2)$ followed immediately by a…
In a video game animation, the geometric center of a character located at $A(1, 3)$ undergoes a translation by vector $\vec{t} = (4, -2)$ followed immediately by a reflection across the point $C(2, 0)$. Find the final position of the character and check analytically whether the result would be the same by applying the reflection first and then the translation.
Two reflections in parallel lines
What happens if you reflect across a line and then across a parallel line? Try it with vertical mirrors $r$: $x = 1$ and $s$: $x = 4$, starting at $P(0, 2)$.
Across $x = 1$, $P(0, 2) \to P'(2, 2)$. Across $x = 4$, $P'(2, 2) \to P''(6, 2)$. The net result is a shift of $\langle 6, 0 \rangle$. The gap between the mirrors is $d = 3$, and the shift is $2d = 6$.
Answer: $\langle -4, 8 \rangle \cdot \langle 2, 1 \rangle = -8 + 8 = 0$, so it is perpendicular. The magnitude is $\sqrt{16 + 64} = 4\sqrt 5$, and the distance between the lines is $\dfrac{10}{\sqrt 5} = 2\sqrt 5$. Twice that is $4\sqrt 5$. ✓
The smallest translation between two parallel lines
If you want a translation that maps one parallel line onto the other, any vector that has the right "perpendicular part" works, but the shortest one is perpendicular to the lines.
Prove analytically that the composition of two reflections in parallel lines is equivalent to a translation. To do this, consider the lines $r \equiv x - 2y = 0$ and…
Prove analytically that the composition of two reflections in parallel lines is equivalent to a translation. To do this, consider the lines $r \equiv x - 2y = 0$ and $s \equiv x - 2y + 10 = 0$. Take a generic point $P(x_0, y_0)$, find its reflection $P'$ in $r$, and then the reflection $P''$ of $P'$ in $s$. Determine the translation vector $\vec{v} = \vec{PP''}$ and check that it is perpendicular to the lines and that its modulus is twice the distance between them.
Given the parallel lines $r \equiv 2x + y - 3 = 0$ and $s \equiv 2x + y + 7 = 0$, find the translation vector $\vec{v}$ with the smallest modulus that transforms the…
Given the parallel lines $r \equiv 2x + y - 3 = 0$ and $s \equiv 2x + y + 7 = 0$, find the translation vector $\vec{v}$ with the smallest modulus that transforms the line $r$ into the line $s$. Prove analytically that this vector is perpendicular to both lines and that its modulus coincides with the shortest distance between them.
Rigid motions and how to identify them
You can often tell which motion you have by comparing the image of a segment $\overline{AB}$ with the segment itself.
| If you find | The motion is |
|---|---|
| $\vec{AA'} = \vec{BB'}$ for all points (so $\vec{A'B'} = \vec{AB}$) | a translation by $\vec{AA'}$ |
| $\vec{A'B'} = -\vec{AB}$, and the midpoints of $AA'$ and $BB'$ coincide | a point reflection through that common midpoint |
An isometry in the plane transforms point $A(1, 2)$ into $A'(5, 4)$ and point $B(3, -1)$ into $B'(7, 1)$. a) Prove that the distance between $A$ and $B$ is the same as…
An isometry in the plane transforms point $A(1, 2)$ into $A'(5, 4)$ and point $B(3, -1)$ into $B'(7, 1)$.
- a) Prove that the distance between $A$ and $B$ is the same as between $A'$ and $B'$.
- b) Determine whether the described transformation corresponds to a pure translation or a reflection in a point, justifying the answer analytically and finding the corresponding translation vector or center of reflection.
Back to the pool table. The cushion is the line $y = k$, so reflect the target ball across it and aim at that image. The cue ball travels in a straight line toward the image, bounces on the cushion at the point where the line crosses it, and the reflected path leads to the real ball. The exercise on this page asks you to find that point.