Power of a lens
Intermediate
Reciprocal of focal length in metres, in dioptres.
Key relation
P = 1/fP = 1/f with f in metres, in dioptres (D). Converging lenses have positive power.
Worked example
f = 25 cm: 4 D.
Common mistake
Glasses prescriptions use dioptres.
In more detail
The power of a lens, P = 1/f, is measured in dioptres when f is in metres. Converging lenses have positive power, and diverging lenses have negative power.
Real-world uses
- Eyeglass prescriptions are given in dioptres.
- Lens combinations add powers to give the overall effect.
- Opticians prescribe glasses in dioptres, where a power of plus two dioptres corresponds to a focal length of half a metre.
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Frequently asked questions
What is a dioptre?
One metre inverse, the power of a lens with a focal length of one metre.
What does a minus power mean?
The lens is diverging, used to correct short-sightedness.
Is a positive power a converging lens?
Yes. Positive power means a converging lens, and negative power means a diverging lens.
Related topics in Optics
Thin lens · Lens formula · Lens maker's formula · Combination of lenses · Lens aberrations · Human eye