principal axis

The term "principal axis" in optics is often used interchangeably with "optical axis." This is a straight line that passes through the geometric center of a lens and connects the centers of curvature of its surfaces. For a curved mirror, the optical axis runs through its geometric center and center of curvature.1

Light rays traveling along the principal axis are perpendicular to the lens or mirror surfaces, so their path remains unchanged. Rays that pass through the lens's optical center are called secondary axes. In the context of geometrical optics and lens theory, the principal axis is crucial for understanding how lenses form images, particularly when considering paraxial rays (rays close to the axis). This concept is fundamental to the Gauss theory of lenses, which simplifies complex lens systems by defining principal and focal points.12