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 Quote by xeno108 I'd really would like to know the equation that relates the kinematics of light under gravity.
From the view of a distant observer the speed of light is reduced by the gravitational field (Shapiro delay):

$u = \frac{c}{{1 - 2 \cdot \frac{\Phi }{{c^2 }}}}$

with

$\Phi \approx - \frac{{\gamma \cdot M}}{{\left| r \right|}}$

The path of the light should be the same as in matter with the same optical density:

$a = 2 \cdot \frac{v}{u} \cdot \left( {v \cdot u'} \right) - u \cdot u'$