Principal Axis:The principal axis is the central axis of a lens or mirror that is parallel to the optical axis and passes through the center of curvature.
Focal Point:The focal point is the point where light rays converge or diverge after passing through a lens or reflecting off a mirror, and is located along the optical axis.
Paraxial Approximation: The paraxial approximation is the assumption that light rays make small angles with the optical axis, allowing for simplified mathematical analysis of optical systems.