The angle by which a light ray is curved by the gravitational field of a massive body. General relativity gives
a value twice as large as that which Newtonian physics would provide, assuming that photons have nonzero mass. The
Newtonian deflection was found by Johann von Soldner as early as 1801 (Lenard 1921; Perlick 2000, p. 200). One of the
first tests of general relativity was a confirmation during the solar eclipse 
of 1919 that the
deflection angle of starlight near the darkened Sun did indeed agree with the relativistic prediction (Pais 1982,
p. 303; Perlick 2000, p. 200). Using very long baseline interferometry, the relativistic value has now been
confirmed within 0.02% (Lebach et al. 1995).
Lebach, D. E.; Corey, B. E.; Shapiro, I. I.; Ratner, M. I.; Webber, J. C.; Rogers, A. E. E.; Davis, J. L.; and Herring, T. A.
"Measurements of the Solar Gravitational Deflection of Radio Waves Using Very Long Baseline Interferometry."
Phys. Rev. Lett. 75, 1439-1442, 1995.
Lenard, P. "Über die Ablenkung eines Lichtstrahls von seiner geradlinigen Bewegung durch die Attraktion eines Weltkörpers, an welchem
er nahe vorbeigeht; von J. Soldner, 1801." Ann. Physik 65, 593-604, 1921.
Pais, A. Subtle Is the Lord: The Science and the Life of Albert Einstein. New York: Oxford University Press, 1982.
Perlick, V. Ray Optics, Fermat's Principle, and Applications to General Relativity. Berlin: Springer-Verlag, 2000.
Weinberg, S. "Unbounded Orbits: Deflection of Light by the Sun."
§8.5 in Gravitation and Cosmology: Principles and Applications of the General Theory of Relativity.
New York: Wiley, pp. 188-194, 1972.
