= Solution
{c}
A <photon> follows a <null geodesic>, whose spatial path approaches straight lines far from the deflecting <mass>. Extend the incoming asymptote forward: this is the reference path the <photon> would follow if the <mass> were removed while its incoming direction and transverse offset were held fixed. The <impact parameter> $b$ is the perpendicular distance from the <mass> to that incoming asymptote. It is not, in general, the distance of closest approach.
\Image[/past-exam-of-the-general-relativity-course-of-the-university-of-oxford/2022-photon-deflection.png]
{title=Impact parameter and deflection relative to the continued incoming photon path}
{description=Original schematic, with bending exaggerated. The dashed horizontal line is the undeflected reference path. The outgoing dashed asymptote makes angle delta with it; b is measured to the incoming asymptote, not to the bent ray.}
The positive deflection angle $\delta$ is the angle between the outgoing propagation direction and the continued incoming direction. The <photon> bends toward the <mass>, so a ray passing above it has a downward outgoing slope in the figure. In the <weak-field approximation> for an isolated spherical <mass>, <Schwarzschild light deflection> gives $\delta\simeq4GM/(bc^2)$ for distant endpoints. This angle differs from the apparent image angle measured from the observer's line of sight to the <mass>.
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