How strong a lens is
A lens can carry a number, like +2.00. What does that number measure?
Step 1 of 3
- A convex lens, f = +0.25 m.
- Its power: P = 1/f = +4 D.
- A concave lens, f = −0.5 m, has power −2 D.
The number is the power of the lens, in dioptres (D). It tells how strongly the lens bends light.
The rule
Power P of a lens is 1/f, with f in metres. Its unit is the dioptre (D). A lens with f = 1 m has a power of 1 D.
Convex lenses have plus power. Concave lenses have minus power. When lenses touch, their powers add.
| f | P = 1/f |
|---|---|
| +0.25 m | +4 D |
| +0.10 m | +10 D |
| +0.50 m | +2 D |
| −0.50 m | −2 D |
| −0.20 m | −5 D |
A shorter focal length gives a bigger power, because P = 1/f. Compare powers without their signs.
Worked example
A lens has f = 25 cm. Change to metres first: 0.25 m. Then P = 1 ÷ 0.25 = +4 D.
Two lenses touch, +2.5 D and −0.5 D. Together P = +2.5 − 0.5 = +2 D. The pair has one focal length. So f = 1/P = 1 ÷ 2 = 0.5 m.
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