e/Optical scalars

Information
has gloss	eng: In general relativity, optical scalars are a set of scalars that describe various properties of null geodesic congruences. The three optical scalars used in general relativity are expansion, shear and twist (vorticity) and were first defined and used by Sachs (1961). Given a vector field k tangent to the null geodesic, the optical scalars are defined as follows.
lexicalization	eng: optical scalars
instance of	c/Mathematical methods in general relativity