Define the principal axis of a lens.

The principal axis of a lens is an imaginary line that passes through the optical center of the lens and is perpendicular to the lens's surface at that point. This axis is a key reference line used in optics to describe the behavior of light as it passes through the lens. The principal axis serves as a baseline for various optical concepts, such as focal points, focal lengths, and image formation.

In addition to the principal axis, lenses have two focal points: one on each side of the lens. Light rays that are parallel to the principal axis either converge to or diverge from these focal points after passing through the lens. The distance between the optical center of the lens and the focal point is known as the focal length of the lens, and it is a fundamental parameter in lens design and analysis.

Understanding the principal axis and its relationship to the focal points is crucial in the study of optics, including the formation of images in optical systems like cameras, telescopes, and eyeglasses.