Page "General relativity" Paragraph 41

This and related predictions follow from the fact that light follows what is called a light-like or null geodesic — a generalization of the straight lines along which light travels in classical physics.

Such geodesics are the generalization of the invariance of lightspeed in special relativity.

As one examines suitable model spacetimes ( either the exterior Schwarzschild solution or, for more than a single mass, the post-Newtonian expansion ), several effects of gravity on light propagation emerge.

Although the bending of light can also be derived by extending the universality of free fall to light, the angle of deflection resulting from such calculations is only half the value given by general relativity.