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(1) Reflection Across the Xaxis
The Axis of Reflection is the Xaxis in the applet below
Move around any red point, to see the effect of reflection across the Xaxis on the blue points:
Each blue point is the reflected, mirror image of the sameletter red point across the Xaxis.
The blue points (A_{2}, B_{2}, C_{2}, D_{2}, E_{2}, F_{2}) automatically react to the movements of its corresponding red point (A_{1}, B_{1}, C_{1}, D_{1}, E_{1}, F_{1}).
In plane geometry, Reflection across a line is defined by moving each point perpendicularly across a straight line (known as the Axis of Reflection), as far into the other side of the given axis, as the distance from the original point to the axis of reflection.
For example, when the axis of reflection is horizontal (like the Xaxis), points originally above it move onto the lower side while points originally below it move onto the upper side. On the other hand, when the axis of reflection is vertical (like the Yaxis), points originally to its right move onto the lefthand side while points originally to its left move onto the righthand side.
Formula for Reflection across the Xaxis
If point P has coordinates P = (x, y), then R = R_{X}(P), its reflected image across the Xaxis, will be defined by
R = (x, y)
For example, the reflected image (across the Xaxis) of point (3, 1), is point (3, 1)
(2) Reflection Across the Yaxis
The Axis of Reflection is the Yaxis in the applet below
Move around any red point, to see the effect of reflection across the Yaxis on the blue points:
Each blue point is the reflected, mirror image of a red point across the Yaxis
The blue points (A_{2}, B_{2}, C_{2}, D_{2}, E_{2}, F_{2}) automatically react to the movements of its corresponding red point (A_{1}, B_{1}, C_{1}, D_{1}, E_{1}, F_{1}).
Formula for Reflection across the Yaxis
If point P has coordinates P = (x, y), then R = R_{Y}(P), its reflected image across the Yaxis, will be defined by
R = (x, y)
For example, the reflected image (across the Yaxis) of point (3, 4), is point (3, 4)
(3) Reflection Across the line { y = x }
The Axis of Reflection is shown in green in the applet below
Move around any red point, to see the effect of reflection across the line y=x on the blue points:
Each blue point is the reflected, mirror image of a red point across the line y = x
The blue points (A_{2}, B_{2}, C_{2}, D_{2}, E_{2}, F_{2}) automatically react to the movements of its corresponding red point (A_{1}, B_{1}, C_{1}, D_{1}, E_{1}, F_{1}).
The line y=x is sometimes called 'The Main Diagonal of the XYplane'
This is the line going through the origin (0,0), with an inclination angle of 45 degrees.
That means the slope of the line is 1, and its Yintercept has a value of 0.
Formula for Reflection across the Main Diagonal { y = x }
Algebraically, in terms of coordinates, you can get the reflected image of a point across the main diagonal simply by switching the x and y values in the coordinates of that point.
If point P has coordinates P = (x, y), then R = R_{D}(P), its reflected image across the line y = x, will be defined by
R = (y, x)
For example, if point P has coordinates (5, 2), then its reflected image across the line y = x
will have coordinates (2, 5).
(4) Reflection Across Any Line
The Axis of Reflection is the line AB shown in green in the applet below
Move around any red point, or the green points A and B, to see the reflection effect on the blue points:
Each blue point is the reflected, mirror image of a red point across the line AB.
The blue points automatically react to the movements of their corresponding red point, and/or to those of the axis of reflection (the green line).
In the interactive Geogebra worksheet above, by individually moving the green points A and/or B, you can move the Axis of Reflection green solid line into any position (vertical, horizontal, or diagonally slanted). This allows you to see the effects of the reflection in each case, because the blue shape is always presented as the reflected image of the red shape, across the axis of reflexion AB.
By moving the red points, you will soon notice this property of reflections: the farther apart from the axis the original points are, the farther apart from the axis their reflected images will be but on the other side of the line. And viceversa, the closer the original points are to the axis of reflection, the closer their reflected images will be to the same axis, just on the other side of it.
Another property of reflections is that the axis of reflection itself remains fixed under the reflection it defines. You will notice this not by moving the axis AB but by leaving it fixed, while bringing onto it any one of the red points. When you place a red point exactly on the green line (the axis of reflection) you will see the corresponding blue point simultaneously moving onto the very same spot.
You may also notice an interesting property that somehow connects reflections with Rotation. When you move around one of the green points (either A or B), the axis of reflection rotates around the other point, the one that remains fixed. At the same time the blue shape, being the reflected image of the red shape across the green line, automatically rotates around the same fixed point (A or B) the axis of reflection is rotating around.
All reflections are examples of geometrical transformations called isometries, because they not only preserve the shapes of geometric figures but also their size.
Some reflections are also Linear Transformations. Not all reflections are linear transformations but only those where the axis of reflection goes through the origin (0,0) of the XYplane. This is because Linear Transformations need to leave the origin fixed. Any geometric transformation of the XYplane that moves the origin (0,0) to somewhere else, is not a linear transformation. However, when the axis of reflection goes through the origin (0,0) of the XYplane, that particular reflection will be a linear transformation.
You may want to try this little activity:
 In the second applet, where the Axis of Reflection is the Yaxis, do the following:
 Place red point D_{1} at coordinates (0, 2)
 Place red point E_{1} at coordinates (3, 4)
 Place red point F_{1} at coordinates (3, 4)
 Now notice how the blue triangle D_{2}E_{2}F_{2}, ended up right on top of the red triangle D_{1}F_{1}E_{1}, both showing as the same triangle, a triangle that is symmetric with respect to the Yaxis (the axis of reflection in the second applet)
This activity illustrates another property of reflections across a line. That is, when a polygon is symmetric with respect to the axis of reflection, its reflected image ends up being the same polygon (with a different ordering of the vertices but still the same subset of the XYplane).

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