= Thin-lens approximation for gravitational lensing
{title2=$\boldsymbol\beta=\boldsymbol\theta-\boldsymbol\alpha(\boldsymbol\theta)$}
The <gravitational lensing> approximation in which deflection is concentrated in a plane whose thickness is small relative to the relevant observer-lens-source distances. The reduced angular deflection includes the distance ratio. A projected <surface mass density> then determines the lensing potential and the image-to-source mapping. This is distinct from a refracting thin optical lens.
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