IGCSE Biology / Coordination
Focusing on near and distant objects revision notes
Edexcel International GCSE Biology (4BI1), chapter 6: Coordination, specification point 2.92. Free lesson.
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Accommodation is how the eye changes the shape of its lens to focus near and distant objects on the retina. The ciliary muscle and suspensory ligaments change the lens shape, which changes how strongly it refracts light.
How the eye focuses on a near object
A near object needs light bent more strongly to focus on the retina. The ciliary muscle is a ring around the lens. When it contracts, the ring gets smaller, so the suspensory ligaments slacken. The elastic lens is no longer pulled flat, so it springs into a fatter, more curved shape. A fatter, more curved lens bends light more strongly. This is called greater refraction.
How the eye focuses on a distant object
For a distant object, the changes are the reverse of those for a near object. The lens needs to bend light less, so it becomes thinner and flatter. The ciliary muscle relaxes and the ring becomes larger. The suspensory ligaments become tight and pull the lens flatter. The thinner, flatter lens bends light less strongly.
Compare near and distant focusing
The change in each part follows a clear pattern. Near focusing makes the lens fatter so it bends light more. Distant focusing makes it thinner so it bends light less. Near: ciliary muscle contracts, ring becomes smaller, ligaments slacken, lens becomes fatter and more curved, light bends more. Distant: ciliary muscle relaxes, ring becomes larger, ligaments become tight, lens becomes thinner and flatter, light bends less.
Why a contracting muscle ring slackens the ligaments
It can seem as if a contracting muscle should pull the ligaments tight. However, the ciliary muscle forms a ring around the lens. When the ring contracts, its diameter decreases and it moves inwards towards the lens. This slackens the ligaments rather than tightening them. Remember the near sequence: contract, slack, fat, bent more. For distant focusing, reverse the sequence: relax, tight, thin, bent less.