1. Structure of the eye
External: the eyeball sits in the bony orbit; eyelids, eyelashes and the conjunctiva (a thin transparent membrane over the front) protect it.
Three coats:
- Sclera — tough, white outer coat; in front it bulges as the transparent cornea, which lets light in and bends it.
- Choroid — dark, with blood vessels; nourishes the retina and absorbs stray light. In front it forms the ciliary body (with ciliary muscles) and the iris, which controls the size of the pupil.
- Retina — innermost, light-sensitive: rods (dim light, no colour; contain visual purple, made with vitamin A) and cones (bright light, colour). The yellow spot has the most cones (sharpest vision); the blind spot, where the optic nerve leaves, has none.
Lens — transparent, biconvex, elastic; held by suspensory ligaments from the ciliary body; focuses light on the retina. Aqueous humour (between cornea and lens) and vitreous humour (jelly behind the lens) keep the shape of the eyeball.
Key terms
- Cornea
Transparent front part of the sclera.
- Iris
Coloured muscular ring that controls the pupil.
- Yellow spot
Part of the retina with most cones; sharpest vision.
- Blind spot
Point where the optic nerve leaves; no rods or cones.
2. Accommodation, adaptation and stereoscopic vision
Accommodation — focusing on objects at different distances by changing the curvature of the lens.
- Near object: ciliary muscles contract → suspensory ligaments slacken → lens thicker, more convex (shorter focal length).
- Distant object: ciliary muscles relax → ligaments tight → lens thinner, flatter.
Adaptation to light and dark:
- Bright light: circular muscles of the iris contract → pupil small; visual purple in rods bleached; cones used.
- Dim light: radial muscles contract → pupil large; visual purple rebuilt (takes several minutes); rods used.
Stereoscopic (binocular) vision: both eyes face forward and their fields overlap (each about 150°, together about 180°, overlap about 120°). The brain fuses the two slightly different views into one three-dimensional image, so we can judge depth and distance.
Key terms
- Accommodation
Adjustment of the lens to focus near and distant objects.
- Stereoscopic vision
Vision with two overlapping fields, giving depth.
- Visual purple
Light-sensitive pigment of the rods (rhodopsin).
3. Defects of the eye
| Defect | Cause | Effect | Correction |
|---|---|---|---|
| Myopia (short sight) | Eyeball too long or lens too curved | Distant objects blurred; image in front of retina | Concave lens |
| Hypermetropia (long sight) | Eyeball too short or lens too flat | Near objects blurred; image behind retina | Convex lens |
| Presbyopia | Lens loses elasticity, ciliary muscles weaken with age | Near objects blurred | Convex or bifocal lens |
| Astigmatism | Uneven curvature of the cornea | Lines in some directions blurred | Cylindrical lens |
| Cataract | Lens becomes cloudy (opaque) | Blurred, dim vision | Surgery: artificial lens |
Ray diagrams are drawn for myopia and hypermetropia only. For myopia, a concave lens whose focal length equals the person's far point makes distant objects appear at that point.
Key terms
- Myopia
Short-sightedness; corrected with a concave lens.
- Hypermetropia
Long-sightedness; corrected with a convex lens.
- Cataract
Clouding of the lens.
4. Structure of the ear
- Outer ear: pinna (collects sound) and auditory canal (with hairs and wax glands that trap dust), ending at the tympanum (ear drum).
- Middle ear: an air-filled cavity with three ossicles — malleus (hammer), incus (anvil), stapes (stirrup; the smallest bone in the body) — joined in a chain from the tympanum to the oval window. The Eustachian tube connects it to the throat and equalises air pressure on both sides of the ear drum.
- Inner ear (membranous labyrinth with fluid): the coiled cochlea, containing the organ of Corti (hearing); the vestibule (utriculus and sacculus); three semicircular canals at right angles, each with a swelling (ampulla) holding sensory hairs. The auditory nerve runs to the brain.
Key terms
- Tympanum
Ear drum; vibrates with sound.
- Ossicles
Malleus, incus and stapes in the middle ear.
- Eustachian tube
Tube joining the middle ear to the throat.
- Cochlea
Coiled part of the inner ear for hearing.
5. Hearing and balance
Path of sound: pinna → auditory canal → tympanum vibrates → malleus → incus → stapes (vibrations transmitted and amplified) → oval window → fluid in the cochlea → sensory hair cells of the organ of Corti produce impulses → auditory nerve → brain (cerebrum), where sound is perceived.
Balance:
- Semicircular canals detect movement and rotation (dynamic balance): turning the head moves the fluid, which bends sensory hairs.
- Vestibule detects the position of the head with respect to gravity (static balance).
- After spinning, the fluid keeps moving for a while, so we feel dizzy.
Key terms
- Organ of Corti
Sensory part of the cochlea for hearing.
- Semicircular canals
Three canals that detect movement of the head.
- Vestibule
Part of the inner ear that senses head position.
Exam tips
Common mistakes
- Saying the ciliary muscles relax for near vision: they contract.
- Swapping the iris muscles: circular for bright light, radial for dim light.
- Confusing the yellow spot (most cones) with the blind spot (none).
- Using a convex lens for myopia; drawing ray diagrams for defects other than myopia and hypermetropia.
- Putting balance in the cochlea.
How marks are usually awarded
- 1 mark: a term (cataract, accommodation), a part named, an odd one out with its category.
- 2 marks: two differences, a mechanism (accommodation, adaptation) in two steps.
- 3 marks: a labelled eye or ear, ray diagrams for myopia or hypermetropia, the path of sound.