Definition of the Principal Axis
The principal axis in optics is an imaginary straight line that serves as the central reference for spherical mirrors and lenses. It passes directly through the optical center of a lens or the center of curvature (and the pole) of a spherical mirror, always perpendicular to the surface of the optical element at its center. This axis simplifies the analysis of light ray paths.
Role in Lenses
For a lens, the principal axis passes through its optical center. Light rays traveling along this axis generally do not deviate, making it a crucial reference. It defines the position of the principal focal points (foci), which are points on the axis where parallel light rays converge after passing through a convex lens, or appear to diverge from after passing through a concave lens.
Role in Mirrors
In the context of spherical mirrors, whether concave or convex, the principal axis connects the pole (the geometric center of the mirror's reflective surface) with the center of curvature. Any light ray incident on the mirror parallel to the principal axis will, after reflection, pass through (for concave) or appear to come from (for convex) the principal focus, which is located on this axis.
Importance in Image Formation
Understanding the principal axis is foundational for constructing ray diagrams, which are graphical tools used to predict the location, size, and orientation of images formed by lenses and mirrors. The predictable behavior of light rays relative to this axis (e.g., parallel rays passing through the focal point) allows for accurate visual representation and calculation of image properties, such as whether an image is real or virtual, inverted or upright.