- Centre of curvature
- Principal axis
- Optical centre
- Radius of curvature
- Focal length
1) Centre of curvature: It is the centre of sphere of which the surface of the lens is a part.
2) Principal axis: It is the line passing through the optical centre of the lens and perpendicular to both the faces of lens.
3) Optical centre: It is the geometrical centre of the lens.
4) Radius of curvature: It is the radius of the sphere of which the surface of the lens is part of.
5) Focal length: It is the distance between the optical centre and the principal focus of a lens.