In physics, the adjective light-like refers to a contour in spacetime in the context of special relativity whose proper length vanishes. A synonym of "light-like" (which is chosen because the worldline of a light ray is an example of a light-like curve) is null (because the proper length equal zero).
This word is used for higher-dimensional manifolds (or surfaces), too, in which case it means that at least one direction at each point is null.
According to general relativity, the gravitational field is coded in a metric of Lorentzian signature on the 4-dimensional spacetimemanifold, and the light rays are the lightlike geodesics of this spacetime metric.
From a mathematical point of view, the theory of gravitational lensing is thus the theory of lightlike geodesics in a 4-dimensional manifold with a Lorentzian metric.
In anisotropic or dispersive media, however, the light rays are not the lightlike geodesics of a Lorentzian metric.
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