centre of curvature

- noun the central point of a circle whose radius is at right angles to a line tangent to any point on the concave side of a smooth curve

- The point at the centre of the sphere formed by the surface of a mirror. Parallel light rays are brought to a focus at a distance of half the radius of curvature with a concave lens, and diverge away from a point half a radius behind a convex one.

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