# Conic constant

In geometry, the conic constant (or Schwarzschild constant,[1] after Karl Schwarzschild) is a quantity describing conic sections, and is represented by the letter K. The constant is given by

${\displaystyle K=-e^{2},}$

where e is the eccentricity of the conic section.

The equation for a conic section with apex at the origin and tangent to the y axis is

${\displaystyle y^{2}-2Rx+(K+1)x^{2}=0}$

where R is the radius of curvature at x = 0.

This formulation is used in geometric optics to specify oblate elliptical (K > 0), spherical (K = 0), prolate elliptical (0 > K > 1), parabolic (K = 1), and hyperbolic (K < 1) lens and mirror surfaces. When the paraxial approximation is valid, the optical surface can be treated as a spherical surface with the same radius.

Some non-optical design references use the letter p as the conic constant. In these cases, p = K + 1.

## References

1. Chan, L.; Tse, M.; Chim, M.; Wong, W.; Choi, C.; Yu, J.; Zhang, M.; Sung, J. (May 2005). Sasian, Jose M; Koshel, R. John; Juergens, Richard C (eds.). "The 100th birthday of the conic constant and Schwarzschild's revolutionary papers in optics". Proceedings of SPIE. Novel Optical Systems Design and Optimization VIII. 5875: 587501. doi:10.1117/12.635041. ISSN 0277-786X.