Optical scalars

In general relativity, optical scalars refer to a set of three scalar functions ${\displaystyle \{{\hat {\theta }}}$ (expansion), ${\displaystyle {\hat {\sigma }}}$ (shear) and ${\displaystyle {\hat {\omega }}}$ (twist/rotation/vorticity)${\displaystyle \}}$ describing the propagation of a geodesic null congruence.

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