Principal Axis
- The line which passes through the centre of the lens and which is perpendicular to the plane of the lens
Optical Centre C
- Point on the principal axis that is the midpoint between the surfaces of the lens
Principal Focal Point F
- The point on the principal axis where all the rays parallel to the principal axis meet after passing through the lens
Focal Plane
- The Plane perpendicular to the principal axis on which all parallel rays meet after passing through the lens. (It is a plane of all the possible focal points)
Focal Length
- Distance between the optical centre C and the principal focus point F.
