The eye is a sense organ — it detects light stimuli and converts them into electrical impulses sent to the brain via the optic nerve.
Key structures:
CORNEA — transparent, curved front surface. Refracts (bends) light — does most of the focusing (about 70% of total refraction).
IRIS — the coloured ring of muscle around the pupil. Controls the size of the PUPIL to regulate how much light enters.
PUPIL — the hole in the centre of the iris. Not a structure itself — just the opening. Lets light into the eye.
LENS — a flexible, transparent disc behind the iris. Fine-tunes focusing by changing shape (ACCOMMODATION). Held in place by suspensory ligaments attached to the ciliary body.
CILIARY MUSCLE — ring of muscle surrounding the lens. Contracts or relaxes to change the tension on the lens via the suspensory ligaments, altering the lens shape and focal length.
SUSPENSORY LIGAMENTS — fibres connecting the lens to the ciliary muscle. When ciliary muscle contracts → ligaments loosen → lens becomes more rounded (for near vision).
RETINA — the light-sensitive layer at the back of the eye. Contains two types of photoreceptor cells: RODS (sensitive to light intensity — monochrome, dim conditions) and CONES (sensitive to colour — need bright light, concentrated in the fovea).
FOVEA (yellow spot) — area of highest cone density on the retina — sharpest colour vision.
OPTIC NERVE — carries electrical impulses from the retina to the brain for processing.
BLIND SPOT — where the optic nerve exits — no photoreceptors here, so no vision in this area.
How the Eye Focuses — Accommodation
ACCOMMODATION is the process by which the lens changes shape to focus on objects at different distances.
FOCUSING ON A NEAR OBJECT:
Ciliary muscles CONTRACT.
Suspensory ligaments SLACKEN (less tension on lens).
Lens becomes more ROUNDED (more curved, fatter).
More refraction → shorter focal length → image focused on retina.
FOCUSING ON A DISTANT OBJECT:
Ciliary muscles RELAX.
Suspensory ligaments become TAUT (pull on lens).
Lens becomes FLATTER (less curved, thinner).
Less refraction → longer focal length → image focused on retina.
MEMORY AID:
Near = ciliary contracts, lens round.
Far = ciliary relaxes, lens flat.
THE PUPIL REFLEX (controlling light entry):
BRIGHT LIGHT → circular muscles of iris CONTRACT → pupil CONSTRICTS (gets smaller) → less light enters → prevents damage to retina.
DIM LIGHT → radial muscles of iris CONTRACT → pupil DILATES (gets larger) → more light enters → improves vision in low light.
⚠️ Common Mistake
For near vision: ciliary muscles CONTRACT and lens becomes more ROUNDED. For far vision: ciliary muscles RELAX and lens becomes FLATTER. Students often get this backwards. Remember: when you look at something NEAR, the ciliary muscle has to work hard (CONTRACT) to make the lens rounder.