Where does the principle axis of a lens always pass through?

The principle axis of a lens is a crucial reference line that defines the optical properties of the lens. It is a straight line that passes through the optical center of the lens, which is the point where light rays passing through the lens either converge or diverge without experiencing any deviation.

The optical center acts as a pivotal point for evaluating the lens's characteristics, such as focal length. When light rays intersect the principle axis at the optical center, they follow a predictable path, which is essential for understanding lens behavior in optical systems. This understanding is fundamental for various applications in optics and vision correction.

The other options, such as the edge of the lens or the circumference, do not serve the same purpose as the optical center in defining the principle axis. The vertex, while related to the geometry of the lens, specifically refers to the point on the lens surface where a line normal to the surface would intersect. Thus, the optical center is the definitive and correct reference point for the principle axis in lens optics.