A convex lens has a focal length of 25 cm. Calculate its power and state its SI unit.
Generated by claude-sonnet-4-6 · 2026-06-26 01:11 · grounding rag
Model Answer
$f = +25\ \text{cm} = +0.25\ \text{m}$
$$P = \frac{1}{f} = \frac{1}{0.25} = +4\ \text{D}$$
The SI unit of power of a lens is dioptre (D).
Source: Light – Reflection and Refraction, Section 9.3.8
---
Explanation
- Convert focal length to metres before applying $P = 1/f$.
- A convex lens has a positive focal length, so its power is positive (+4 D).
- Always state the unit (dioptre, D) since the question asks for it — in a 1-mark question, both the value and the unit are expected on one line or two very short lines.