Geodesic Deviation & Jacobi Fields

The Jacobi equation D²J/ds² + R(J,T)T = 0 governs how nearby geodesics separate. On a sphere (K>0) they converge; on a saddle (K<0) they diverge exponentially; flat space gives parallel lines.

Curvature K
J(s) at s=π/2
Surface type
Conjugate point s