| English | Thin lens formula |
|---|---|
| Bahasa Melayu | Rumus kanta nipis |
| 中文 | 薄透镜公式 |
Definition
The relationship linking object distance u, image distance v and focal length f of a thin lens: 1/f = 1/u + 1/v.
In exam terms
The thin lens formula links the object distance u, the image distance v and the focal length f of a thin lens: 1/f = 1/u + 1/v. All three distances must be in the same unit (m or cm).
For a converging lens, f is positive. For example, if u = 30 cm and f = 10 cm, then 1/v = 1/10 − 1/30 = 2/30, so v = 15 cm.
A positive v means a real image forms on the far side; a negative v means a virtual image on the same side as the object.
Do not confuse it with the magnification formula
Do not confuse the thin lens formula with the magnification formula m = v ÷ u. The lens formula finds where the image is; the magnification formula finds how big it is.
They are used together in most numerical questions.
In Paper 2, a calculate the image distance question expects clear working with 1/f = 1/u + 1/v and correct sign, while a follow-up part often asks you to state whether the image is real or virtual based on the sign of v.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)