= Solution
A <light> ray follows a <null geodesic>. For a weak, approximately spherical field and <impact parameter> $b\gg GM/c^2$, the asymptotic <Schwarzschild light deflection> is
$$
\boxed{\Delta\theta=\frac{4GM}{bc^2}.}
$$
At the solar limb this is about $1.75$ arcseconds, whereas a simple Newtonian fast-particle calculation gives half that value. The difference tests relativistic <spacetime> geometry beyond the universality of slow-body free fall. The quoted <angle> compares incoming and outgoing directions far from the deflector, not a locally measured turn at a sharp vertex.
\Image[/past-exam-of-the-relativity-course-of-the-university-of-cambridge/2016/paper-1-light-bending.png]
{title=Original weak-deflection sketch with incoming direction, curved ray and apparent sky direction; the angle is exaggerated}
For an eclipse measurement, photograph background stars near the obscured <Sun> and compare their apparent positions with reference plates taken when that field is away from the <Sun>. Back-projecting the deflected incoming <light> makes a star appear displaced outward from the <Sun>'s apparent position. Fit the radial displacement against inverse angular separation from the <Sun>, while allowing plate translation, rotation and scale. The expected pattern must be separated from <atmospheric refraction>, <optical distortion> and changes of focus or scale, rather than inferred from one displaced star.
The 1919 expeditions obtained observations at Sobral and Príncipe, published by <Frank Watson Dyson>, <Arthur Eddington> and <Charles Davidson> in 1920. The original report compares eclipse and reference plates and discusses instrument scale and image-quality limitations; it should not be treated as an exact modern precision measurement.<A determination of the deflection of light by the Sun's gravitational field by Frank Watson Dyson and others>[pp. 291–333, observing arrangements, reference photographs, plate reductions and final deflection comparisons]{ref} Repeated observations with independent instruments and calibrated reference fields test whether the displacement follows the predicted geometry. <vacuum> gravitational bending is achromatic to this order, a useful distinction from dispersive propagation effects.
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