Specialised vocabulary and subject sections

A thin converging lens

A thin converging lens is a transparent object (made of glass or plastic) that is thicker at the centre than at the edges. It bends the rays of light passing through it so that they converge at a single point.

Key elements

• The optical centre O: the point at the centre of the lens through which light passes without being deflected.
• The optical axis: a horizontal line passing through O, perpendicular to the plane of the lens.
• The image focus F': the point on the optical axis where all rays initially parallel to the axis converge.
• The object focus F: the point symmetrical to F’ with respect to O.
• The focal length f’: the distance OF’ (in metres), which is always positive for a converging lens.

The three special rays

To construct an image, we use rays whose exact path is known:

Incident ray	Behaviour after the lens
Passes through the optical centre O	Is not deflected, continues in a straight line
Parallel to the optical axis	Emerges through the image focus F'
Passes through the object focus F	Emerges parallel to the optical axis

Practical example

If f' = 5 cm, a ray parallel to the axis that strikes the lens 3 cm above O will emerge in the direction of F', situated 5 cm from O on the axis. These three rays are sufficient to precisely locate the image of a point source of light.

Common pitfall

Do not confuse the object focus F with the image focus F’: they are symmetrical about O but play different roles in the construction. F’ relates to rays arriving parallel to the optical axis; F relates to rays emerging parallel to the optical axis.