Mirror line as line of symmetry
If a figure can be folded along a line such that both the parts coincide, then . . .
. . . the figure is said to be
$symmetrical$
along that fold line and . . .
. . . this line is called the line of symmetry.
Let's cut this figure along the line of symmetry.
If we place this part on the mirror, its reflection completes the figure.
The mirror here acts as a line of symmetry for the complete figure.
In a figure with a line of symmetry, one half is called the mirror image of its other half.
So, the mirror line is the same as the line of symmetry.
Reflection by a mirror can help us find the line of symmetry.
Let's see another case.
Let's fold a piece of paper in half.
If we punch holes into the folded piece of paper, . . .
. . . and then unfold it, we get the holes in both parts of the paper.
These holes are symmetric along the fold line.
This fold line is the line of symmetry or the mirror line.
Revision
Line of symmetry divides a figure into two halves, which coincide with each other.
If we bring one half of the figure in front of a mirror, its reflection is the other half.
The mirror line is the same as the line of symmetry.
A mirror can help us in visualizing the line of symmetry for a figure.
The End