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Learn7408Paper 3B · Option: Medical physicsPhysics of the eyeThe eye and lenses
Physics of the eye · Subtopic 1 of 1

The Eye and Lenses

The eye focuses light on the retina mainly with the cornea, fine-tuned by the lens. Lenses obey 1f=1u+1v\frac1f=\frac1u+\frac1v and have power P=1fP=\frac1f in dioptres (D). A normal eye sees from 2525 cm (near point) to infinity (far point). Short sight (myopia) is corrected with a diverging lens; long sight (hypermetropia) with a converging lens; astigmatism with a cylindrical lens.

AQA 7408 Paper 3B≈ 40 minRevision 1 · updated 2026-10-27
Section 1 of 14

Learning objectives

  • Use 1f=1u+1v\frac1f=\frac1u+\frac1v and P=1fP=\frac1f.
  • Describe rods and cones and the eye's spectral response.
  • Calculate correcting lens powers for myopia and hypermetropia.
Pause and think

Why does everything look blurry when you open your eyes underwater?

Section 2 of 14

Core idea

Start hereA living camera

Your eye refocuses from a page to a distant hill in a fraction of a second, by changing the shape of a lens. When that system is slightly off, a thin piece of glass with the right power puts everything back in focus.

The big idea

1f=1u+1v\frac1f=\frac1u+\frac1v, P=1fP=\frac1f. A correcting lens takes an object where you want to see it and forms a virtual image where your eye can focus.

1. Lenses

1f=1u+1v,P=1f\frac1f=\frac1u+\frac1v,\qquad P=\frac1f
where
u,vu,v
object and image distances (virtual image: v negative).
PP
power in dioptres, f in metres.
Worked exampleA real image

Object 0.300.30 m from a lens with f=0.20f=0.20 m.

Goal
vv.
Strategy
Lens equation.
  1. 1v=10.20−10.30=1.67\frac1v=\frac1{0.20}-\frac1{0.30}=1.67
    Rearrange.
  2. v=0.60 mv=0.60\text{ m}
    Invert.
Answer0.600.60 m, real and inverted.
✓ CheckObject between ff and 2f2f gives image beyond 2f2f ✓.

2. Correcting vision

Lens for myopia
  1. u=∞u=\infty
    We want to see distant objects.
  2. v=−far pointv=-\text{far point}
    Virtual image where the eye can focus.
  3. P=1u+1v=−1far pointP=\frac1u+\frac1v=-\frac1{\text{far point}}
    Negative: diverging.
Far point 2.02.0 m → P=−0.50P=-0.50 D.
Worked exampleLong sight

Near point 1.01.0 m; read at 0.250.25 m.

Goal
Lens power.
Strategy
u=0.25u=0.25, v=−1.0v=-1.0.
  1. P=10.25+1−1.0P=\frac1{0.25}+\frac1{-1.0}
    Lens equation.
  2. =+3.0 D=+3.0\text{ D}
    Converging.
Answer+3.0+3.0 D converging lens.
✓ CheckLong sight needs extra converging power ✓.
Defects of vision

Match the defect to the lens.

Key takeawayRods and cones Rods: sensitive, monochrome, dim light. Cones: red, green, blue, colour and fine detail, concentrated in the fovea.
Common misconceptionSigns A virtual image has negative vv; a diverging lens has negative power.
Pause and think

Why does reading become harder after age 45?

Summary

  • 1f=1u+1v\frac1f=\frac1u+\frac1v.
  • Myopia: P=−1far pointP=-\frac1{\text{far point}}.
  • Hypermetropia: converging.
Key words
Dioptre
Unit of lens power, m⁻¹.
Near point
Closest point of clear vision.
Far point
Furthest point of clear vision.
HistoryReading stones. Monks around 1000 AD read with polished glass "reading stones". Spectacles appeared in Italy in the 1280s.
Fun factYour blind spot. Where the optic nerve leaves the retina there are no rods or cones; your brain fills in the gap so you never notice.
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