| Principal
Focus |
Line joining the centre of
curvatures of 2 surfaces. It passes through the optical centre and the foci of the lens. |
| Optical
Centre |
Centre of the lense through
which all will pass through undeviated. |
| Principal
Focus |
Point on the principal axis to
which a beam of light, close and parallel to the principal axis will converge after
passing through the lens. |
| Focal
Length |
Distance between the optical
centre and the principal focus (P). |
