Define the power of a thin lens.

The power of a lens is the reciprocal of its focal length, P = 1/f, and it is the curvature the lens adds to a wave-front.

Why is the power of a converging lens taken as positive?

It adds positive curvature and makes wave-fronts more convergent, whereas a diverging lens removes curvature and is given a negative power.

Write the thin lens relationship in curvature form and say what each term means.

1/v = 1/u + 1/f, where 1/u is the curvature of the wave-front arriving at the lens, 1/f is the curvature the lens adds and 1/v is the curvature of the wave-front leaving.

OCR A-Level Physics B H557

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Imaging and Signalling

Lens power and focal length

3 things to know
Components 01, 02 and 03

Overview

Recall questions on lens power P = 1/f and the dioptre, converting between power and focal length, the positive power of a converging lens, the thin lens relationship in curvature form, adding the powers of lenses in contact, why opticians use dioptres, the curvature a lens adds to a wave-front, and how refractive index and surrounding water change power.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

State the unit of lens power and its SI equivalent.

Numbers

Calculate the power of a converging lens of focal length 25 cm.

Numbers

A lens has a power of +8.0 D. Calculate its focal length.

Questions from this subtopic

Each one with the model answer underneath.

1
Define the power of a thin lens.
Model answer
The power of a lens is the reciprocal of its focal length, P = 1/f, and it is the curvature the lens adds to a wave-front.
2
State the unit of lens power and its SI equivalent.
Model answer
Lens power is measured in dioptres (D), and 1 D is the same as 1 m⁻¹.
3
Calculate the power of a converging lens of focal length 25 cm.
Model answer
P = 1/0.25 = +4.0 D.
4
A lens has a power of +8.0 D. Calculate its focal length.
Model answer
f = 1/8.0 = 0.125 m, which is 12.5 cm.
5
Why is the power of a converging lens taken as positive?
Model answer
It adds positive curvature and makes wave-fronts more convergent, whereas a diverging lens removes curvature and is given a negative power.
6
Write the thin lens relationship in curvature form and say what each term means.
Model answer
1/v = 1/u + 1/f, where 1/u is the curvature of the wave-front arriving at the lens, 1/f is the curvature the lens adds and 1/v is the curvature of the wave-front leaving.

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