For a spherical mirror, any rays which are parallel to the principal axis will be reflected through the focal point, which is half way between the centre of curvature and the surface of the mirror. ie. the focal point is half the radius away from the centre of curvature.

shallot points out that the focal point can be brought closer to the spherical mirror by filling it with a substance that has a higher index of refraction. I guess this would only work if your rays were coming from directly above.

For a lens, the focal point is a point where any incident ray which is parallel to the principal axis will pass through.

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