Reflection
Reflections are everywhere ... in mirrors, glass, and here in a lake.
... what do you notice ?
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Every point is the same distance from the central line !
... and ...
The reflection has the same size as the original image.
The central line is called the Mirror Line:
Can A Mirror Line Be Vertical?
Yes. Here my dog "Flame" shows a Vertical Mirror Line (with a bit of photo editing).
In fact Mirror Lines can be in any direction.
Imagine turning the top image in different directions:
A reflection is a flip over a line
You can try reflecting some shapes about different mirror lines here:
"How Do I Do It Myself?"
Just approach it step-by-step. For each corner of the shape:
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Labels
It is common to label each corner with letters, and to use a little dash (called a Prime) to mark each corner of the reflected image.
Here the original is ABC and the reflected image is A'B'C'
Some Tricks
X-Axis
When the mirror line is the x-axiswe change each (x,y) into (x,−y)
Y-Axis
When the mirror line is the y-axis
we change each (x,y) into (−x,y)
Fold the Paper
And when all else fails, just fold the sheet of paper along the mirror line and then hold it up to the light !