The following technical terms occur in discussions of lenses.

The curvature of each surface of a lens makes it part of a sphere. The centre of curvature of the lens is the centre of this sphere.

The principle axis is the line through the centre of the lens, perpendicular to the lens. The lens has two surfaces – focal points and centres of curvature for both sides of the lens lie on this line.

The focal point or principal focus F is the point on the principal axis through which rays of light parallel to the axis are brought to a focus after passing through the lens (converging lens) or appear to be brought to a focus (diverging lens).
As with centres of curvature, there is a focal point on either side of the lens.

The focal length is the distance between the centre of the lens and a focal point.

The distance from the centre of the lens to a centre of curvature is twice the focal length.

The linear magnification is the ratio between the size of the image and the size of the object, also equal to the ratio of the image and object distances:
This is easily seen from the diagram below. The triangles OAB and BCI are similar, soand which rearranges to (1) .
