Theory:

The lines of symmetry and the mirror images are linked to each other.
 
When we fold a piece of paper into two halves, then the folded line becomes the mirror line.
 
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What does a mirror do?

A mirror reflects an image from left to right. The reflection depends on the projection of the picture. In other words, whatever portion (partial or whole) we place in front of the mirror, it reflects.
 
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Similarly, whatever is on one side of the line of symmetry will be reflected on the other side of the line of symmetry. This mirror-like symmetry is called reflection symmetry.
 
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Reflection symmetry:
Imagine a mirror being placed on the line of symmetry. The half, projected on the mirror, is completed by its reflection on the mirror. This symmetry is called reflection symmetry or mirror symmetry.
The objects and its reflection look identical. The only difference is that the object looks flipped horizontally on the mirror.
 
Let us look at a few more examples.
 
Object
Reflection
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