Structure of the human eye
The eyeball is nearly round, about 2.3 cm across. Its main parts, in the order light meets them:
- Cornea: the clear, bulging front window. Most of the bending (refraction) of light happens here.
- Iris: the coloured ring. It controls the size of the pupil, the dark hole in its centre, to let in more light in the dark and less in bright light.
- Eye lens: a clear, jelly-like convex lens. It fine-tunes the focus.
- Ciliary muscles: hold the lens and change its curvature.
- Retina: the light-sensitive screen at the back with rod and cone cells. They turn light into electrical signals.
- Optic nerve: carries the signals to the brain, which reads the upside-down image and shows it to us the right way up.
So the eye works as a lens system: cornea + lens make a real, inverted, small image on the retina.
Power of accommodation
The distance from the lens to the retina cannot change, so the eye changes the focal length of its lens instead.
- Far object → ciliary muscles relax → lens thin → focal length longest.
- Near object → ciliary muscles contract → lens thick → focal length shorter.
This ability is the power of accommodation. It has limits:
- Near point (least distance of distinct vision): about 25 cm for a young, normal eye.
- Far point: infinity for a normal eye.
Why the pupil changes size
In a dark room the pupil widens to collect more light; step into sunlight and it shrinks. The iris does this by itself.
Defects of vision and their correction
Myopia (near-sightedness)
Near things are clear, far things are blurred. The far point comes closer than infinity. Causes: the eyeball is too long, or the lens is too curved (too much converging power). The image of a far object forms in front of the retina. Correction: a concave (diverging) lens of suitable power, focal length = −(far point).
Hypermetropia (far-sightedness)
Far things are clear, near things are blurred. The near point moves farther than 25 cm. Causes: the eyeball is too short, or the lens focal length is too long. The image of a near object would form behind the retina. Correction: a convex (converging) lens.
Presbyopia
With age the ciliary muscles weaken and the lens becomes less flexible, so the near point moves away. Older people need reading glasses (convex). Some have both myopia and presbyopia and use bifocal lenses: concave part on top for distance, convex part below for reading.
Other fixes
Contact lenses and eye surgery can also correct these defects. A cloudy lens in old age is a cataract, treated by surgery.
Why two eyes, and board-exam tips
Two eyes give a wider field of view and help judge depth (3D vision), because each eye sees a slightly different picture.
Exam pattern: 2–3 mark questions on labelled ray diagrams of myopia/hypermetropia with correction, a numerical on lens power (P = 1/f in metres), and 1-mark questions on near point, far point, accommodation and the function of the iris/pupil.
Key formulas and definitions
- Power of a lens: P = 1 / f (f in metres), unit dioptre (D)
- Myopia correction: concave lens with f = −(far point of the defective eye)
- Hypermetropia correction: 1/f = 1/0.25 − 1/(near point in m), convex lens
- Normal eye: near point ≈ 25 cm, far point = infinity
- Concave lens → negative power; convex lens → positive power
Worked examples
1. A myopic person cannot see clearly beyond 2 m. What lens and power are needed?
Far point = 2 m. A concave lens must make parallel rays look as if they come from 2 m, so f = −2 m. P = 1/f = −0.5 D. Concave lens of −0.5 D.
2. A student's far point is 80 cm. Find the power of the correcting lens.
f = −80 cm = −0.8 m. P = 1/(−0.8) = −1.25 D (concave).
3. A doctor prescribes a lens of −2.5 D. What is the defect and the far point?
Negative power means a concave lens, so the defect is myopia. f = 1/P = 1/(−2.5) = −0.4 m. The far point is 40 cm.
4. A hypermetropic eye has its near point at 1 m. What lens lets it read at 25 cm?
The lens must form a virtual image at 1 m of a book at 25 cm. Using 1/f = 1/v − 1/u with u = −25 cm, v = −100 cm: 1/f = −1/100 + 1/25 = 3/100, so f = 33.3 cm = 0.333 m. P = +3 D (convex).
5. The near point of an old man is 50 cm. Find the power of his reading glasses.
u = −25 cm, v = −50 cm. 1/f = 1/v − 1/u = −1/50 + 1/25 = 1/50, f = 50 cm = 0.5 m, P = +2 D.
6. A person needs −1.5 D for distance and +2 D for reading. What spectacles should she use?
She has both myopia and presbyopia, so she needs bifocal lenses: the upper part concave (−1.5 D, f ≈ −67 cm) for distance and the lower part convex (+2 D, f = 50 cm) for reading.
Common mistakes
- Saying the eye changes the distance between lens and retina. It changes the focal length (thickness) of the lens.
- Mixing up the lenses: myopia needs a concave (minus) lens; hypermetropia and presbyopia need a convex (plus) lens.
- Forgetting to convert cm to metres before using P = 1/f.
- Saying ciliary muscles contract for far objects. They relax for far objects and contract for near ones.