The Human Eye and the Colourful World
The human eye and defects of vision
Learn how the eye focuses and how spectacles correct vision.
Your eye works like a camera. The eyeball is roughly a ball about 2.3 cm across. Follow the light in:
| Part | Job |
|---|---|
| Cornea | the clear bulge at the front; light enters here, and most of the bending happens here |
| Iris | the coloured ring; a muscle that controls the size of the pupil |
| Pupil | the dark opening that lets light in |
| Eye lens | fine-tunes the focus; forms a real, inverted image |
| Retina | the screen at the back; its cells send signals to the brain along the optic nerve |
"If the hole gets smaller, does more light go in, or less?" Less. In bright light the iris makes the pupil small; in dim light it opens it wide.
Unlike the hard lens of spectacles, the eye lens is soft and jelly-like, so its shape can change. The ciliary muscles do this. This is accommodation:
- looking at something far away: the muscles relax, the lens becomes thin, its focal length gets longer;
- looking at something near: the muscles contract, the lens becomes thicker, its focal length gets shorter.
That is why reading for a long time tires your eyes, but gazing at the distance does not.
There's a limit. Hold a page very close and it blurs. The closest you can see clearly without strain is the near point, about 25 cm for a young adult. The farthest is the far point, which is infinity for a normal eye.
Defects of vision and their correction:
| Defect | Can't see clearly | Image forms | Corrected with |
|---|---|---|---|
| Myopia (near-sightedness) | far objects | in front of the retina | a concave lens |
| Hypermetropia (far-sightedness) | near objects | behind the retina | a convex lens |
How does the concave lens help? It makes a far-away object appear to be at the eye's own far point, where the eye can see it. The convex lens does the opposite for a near object.
Presbyopia: with age, the ciliary muscles weaken and the lens grows stiff, so near vision becomes hard. Some people have both defects and need bifocals: the upper part is concave (for distance), the lower part convex (for reading). In old age the lens can also turn milky and cloudy: a cataract. Vision can be restored by cataract surgery.
Eye donation, from the book. Our eyes can help someone see even after we die. Donated corneas can cure corneal blindness, and one pair of eyes can give sight to up to four people. Anyone can donate, of any age or sex, even people who wear spectacles or have had a cataract operation. The eyes must be removed within 4–6 hours after death: the family informs the nearest eye bank, removal takes only 10–15 minutes and leaves no disfigurement, and the identities of donor and receiver are kept private.
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