Your Eye Lens Changes Shape Every Time You Look Up From Your Book

Look up from this page to a tree outside the window and both are sharp within a moment. Walk from bright sunlight into a dim room and after a short while you can see again.

This part covers the structure of the eye, accommodation and adaptation, stereoscopic vision, and the common defects of vision with their corrections.

The eye has three layers — sclera, choroid and retina — plus a lens and two fluids; together they focus light onto the retina, which sends impulses to the brain.

Protective parts: eyelids and eyelashes keep out dust, and tears keep the eye moist and kill germs.

Internal parts:

- Sclera — tough white outer layer that protects the eyeball
- Cornea — transparent front part of the sclera; bends light as it enters
- Choroid — dark middle layer with blood vessels; its pigment stops light reflecting inside the eye
- Iris — coloured ring from the choroid; controls the size of the pupil, the opening that lets light in
- Lens — transparent and biconvex, held by suspensory ligaments from the ciliary body; focuses light
- Aqueous humour — watery fluid in front of the lens; vitreous humour — jelly behind it, keeping the eyeball's shape
- Retina — inner light-sensitive layer with rods for dim light and cones for colour and bright light
- Yellow spot — part of the retina with most cones; sharpest vision
- Blind spot — where the optic nerve leaves; no rods or cones, so no image forms there

The image on the retina is real, inverted and smaller; the brain interprets it upright.

An everyday example. Eye donation after death helps others see, because a clear donated cornea can replace a damaged one.

Accommodation is the lens changing its thickness to focus near or distant objects, adaptation is the eye adjusting to bright or dim light, and stereoscopic vision is the brain combining the slightly different views of two eyes to judge depth.

1. Accommodation

- Distant object — ciliary muscles relax, suspensory ligaments pull tight, lens becomes thinner
- Near object — ciliary muscles contract, ligaments slacken, lens becomes thicker and bends light more
- A normal young eye sees clearly from about $25\text{ cm}$, the near point, to very far away

2. Adaptation

- Bright light — iris muscles make the pupil constrict, letting in less light
- Dim light — the pupil dilates and rods slowly rebuild their light-sensitive pigment

3. Stereoscopic vision. Both eyes face forward, so their fields of view overlap. Each eye sees the object from a slightly different angle, and the brain merges the two images into one, giving a sense of depth and distance.

An everyday example. Try threading a needle with one eye closed — judging where the thread is becomes surprisingly hard.

The substance. Adapting to dim light is much slower than adapting to bright light, which is why a dark hall leaves you stumbling at first.

Myopia and hypermetropia come from an eyeball of the wrong length or a lens of the wrong curvature, presbyopia from an ageing lens, astigmatism from uneven curvature, and cataract from a cloudy lens.

- Myopia (short-sightedness) — distant objects blurred; eyeball too long or lens too curved, so the image forms in front of the retina
- Hypermetropia (long-sightedness) — near objects blurred; eyeball too short or lens too flat, so the image would form behind the retina
- Presbyopia — in middle and old age the lens loses its elasticity and ciliary muscles weaken, so near vision becomes difficult
- Astigmatism — the cornea or lens is unevenly curved, so lines in some directions look blurred
- Cataract — the lens becomes cloudy, often with age, so vision grows hazy

An everyday example. A grandparent holding a newspaper at arm's length to read it is showing presbyopia.

The misconception. Myopia does not mean weak eyes that see nothing clearly — a myopic person sees near objects well; only distant ones are blurred.

Myopia is corrected with a concave lens, hypermetropia and presbyopia with a convex lens, astigmatism with a cylindrical lens, and cataract by surgically replacing the cloudy lens.

Ray diagram for myopia. Draw parallel rays from a distant object meeting in front of the retina. Then place a concave lens before the eye: it spreads the rays slightly so they meet on the retina.

Ray diagram for hypermetropia. Draw rays from a near object meeting behind the retina. Then place a convex lens before the eye: it converges the rays so they meet on the retina.

Worked example — myopia. A person's far point is $2\text{ m}$. The correcting concave lens has $f=-2\text{ m}$:

$$P=\frac{1}{f}=\frac{1}{-2}=-0.5\text{ D}$$

Worked example — hypermetropia. A near point of $50\text{ cm}$ must be brought to $25\text{ cm}$. With $u=-25\text{ cm}$ and $v=-50\text{ cm}$:

$$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}=-\frac{1}{50}+\frac{1}{25}=\frac{1}{50}f=+50\text{ cm}=0.5\text{ m}P=+2\text{ D}$$

Other corrections: bifocal lenses for presbyopia; cylindrical lenses for astigmatism; for cataract, the cloudy lens is removed and an artificial lens placed inside the eye.

An everyday example. Free eye camps in towns and villages commonly test vision and arrange cataract operations.

The substance. A negative power always means a concave lens, used for myopia.