The eye (HT): structure, accommodation and common defects

The eye is a sense organ containing receptors sensitive to light intensity and colour. Higher-tier learners need to know its structure, the iris and accommodation reflexes, and how myopia and hyperopia are corrected.

Structure (label these in the exam)

• Cornea — clear front layer that does most of the focusing.
• Iris — coloured ring of muscle that controls how much light enters by changing pupil size.
• Pupil — the hole in the iris through which light enters.
• Lens — flexible disc behind the iris that fine-tunes focusing onto the retina.
• Ciliary muscles — ring of muscle holding the lens; contract or relax to change lens shape.
• Suspensory ligaments — fibres connecting the ciliary muscles to the lens.
• Retina — light-sensitive layer at the back containing rods and cones.
• Optic nerve — carries impulses from the retina to the brain.
• Sclera — tough white outer coat that protects the eye.

Iris reflex (controlling light intensity)

The pupil dilates or constricts to protect the retina:

• Bright light → circular muscles in the iris contract, radial muscles relax → pupil constricts (smaller).
• Dim light → radial muscles contract, circular muscles relax → pupil dilates (larger), letting more light in.

This is a reflex action — fast, automatic, no thought required.

Accommodation (focusing on near vs far)

The lens shape changes to focus light onto the retina from objects at different distances. This is called accommodation.

To focus on a near object:

• Ciliary muscles contract.
• Suspensory ligaments slacken (loosen).
• Lens becomes more rounded / fatter.
• Light is refracted (bent) more strongly to bring it to focus on the retina.

To focus on a distant object:

• Ciliary muscles relax.
• Suspensory ligaments are pulled tight.
• Lens is pulled thin / less curved.
• Light is refracted only slightly.

A common confusion: when ciliary muscles contract, ligaments slacken (not tighten). Imagine a ring of muscle around a stretched rubber band — when the ring shrinks, the band becomes loose.

Common defects and their correction

Myopia (short-sightedness) The person can see near objects clearly but distant objects appear blurry. Either the lens is too curved or the eyeball is too long, so distant light is focused in front of the retina. Corrected with a concave (diverging) lens that spreads the light out before it reaches the eye, so the focal point moves back onto the retina.

Hyperopia (long-sightedness) Near objects are blurry, distant objects fine. Either the lens is not curved enough or the eyeball is too short, so the focus point falls behind the retina. Corrected with a convex (converging) lens that bends light inwards, bringing the focus forward onto the retina.

Other modern corrections (just need to be aware of them): contact lenses (sit on the cornea), laser eye surgery (changes the cornea's curvature), replacement lens surgery (for cataracts).

Required diagrams

Be able to:

• Label all the eye structures listed above.
• Draw a ray diagram showing parallel rays focusing in front of (myopia) or behind (hyperopia) the retina, and the same diagram with a corrective lens added.

Links

Builds on B5.2 (sensory pathways via the optic nerve). Connects to physics topics on lenses and refraction.