| English | Focal length |
|---|---|
| Bahasa Melayu | Jarak fokus |
| 中文 | 焦距 |
Definition
The distance between the optical centre of a lens (or mirror) and its focal point.
In exam terms
The focal length f is the distance between the optical centre of a lens (or mirror) and its focal point (principal focus), where rays parallel to the principal axis converge or appear to diverge from. It is measured in metres (m) or centimetres (cm).
A shorter focal length means a more powerful, more strongly curved lens. A correct sentence: the converging lens has a focal length of 10 cm, so parallel rays meet 10 cm behind it. Focal length is used in the thin lens formula 1/f = 1/u + 1/v.
Do not confuse it with object or image distance
Do not confuse focal length f with the object distance u or image distance v. Focal length is a fixed property of the lens; u and v change with where the object is placed.
Only when the object is very far away does v equal f.
In Paper 2, an experiment question may ask you to describe how to find the focal length by focusing a distant object onto a screen and measuring the distance, while a calculate question uses 1/f = 1/u + 1/v with values in the same unit.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)