The principal axis of a lens is an imaginary straight line passing through the two centres of curvature (C1 and C2). It is a key reference line used to describe the geometry and behavior of the lens.

The optical centre of a lens, represented by the letter O, is the central point of the lens. A ray of light passing through the optical centre does not undergo any deviation. It is a crucial point for understanding the ...

The aperture of a lens is the effective diameter of its circular outline. In this chapter, we focus on lenses with small apertures, where the aperture is much less than the radius of curvature, and the two centres of curvature ...