What is the definition of the principal axis of a spherical mirror?

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The principal axis of a spherical mirror is a line going through the center of the mirror that is exactly perpendicular (sometimes said "normal") to the surface of the mirror. By definition, a spherical mirror is rotationally symmetrical around its principal axis.

Why is this useful? Well, for one thing, light rays coming into the concave side of a spherical mirror that are parallel to the principal axis will be reflected into a focal point that itself lies on the principal axis. The further the light rays are from the principal axis, the closer to the mirror that focus will lie---so not all light from a spherical mirror is focused in the same place, which is called spherical aberration.

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