A plane that cuts through a molecule in a way that images of all the molecule's features beyond the plane seem to produce an identical molecule is a mirror plane or a plane of reflection. The corresponding operation as well as the plane as symmetry element are denoted with the greek letter σ.
A twofold reflection on one plane produces the operation of identity: σ2 ≡ E. Any planar molecule has at least one mirror plane.
The orientation of a mirror plane relative to the molecule's main axis is indicated by a subscript. σh indicates a plane which is perpendicular to this axis or horizontal, whereas σv is the symbol for vertical mirror planes containing the main axis. If such a plane bisects the angle between a pair of rotational axis C2, we have a diagonal mirror plane σd.
|Fig. 1: Three types of mirror planes: σv, σh, σd|
|Representative: molecule H2O featuring two vertical mirror planes σv|
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|Fig. 2: In the water molecule, the two planes of reflection intersect in axis C2.|
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