Optical scalars
In general relativity, optical scalars refer to a set of three scalar functions { θ ^ {\displaystyle \{{\hat {\theta }}} (expansion), σ ^ {\displaystyle {\hat {\sigma }}} (shear) and ω ^ {\displaystyle {\hat {\omega }}} (twist/rotation/vorticity) } {\displaystyle \}} describing the propagation of a geodesic null congruence. In fact, these three scalars { θ ^ , σ ^ , ω ^ } {\displaystyle \{{\hat {\theta }}\,,{\hat {\sigma }}\,,{\hat {\omega }}\}} can be defined for both timelike and null geodesic congruences in an identical spirit, but they are called "optical scalars" only for the null case.