The principal axis of a lens is the line along which light rays converge.
Light rays that pass through a lens converge at a point known as the focal point. The focal length of a lens is measured from the center of the lens to this point. The distance between the principal axis and the focal point is known as the optical axis, which is perpendicular to the principal axis. The principal axis lies in front of and behind a convex or concave lens, respectively.
What is an optical axix?
Light rays that pass through a convex or concave lens converge at a point known as the focal point. The distance between this focal point and where it intersects with an imaginary line drawn perpendicular to both sides of the lens (known as its principal axis) is known as its optical axis.
