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Coordination and Response: Question 8

Syllabus 14.2

Structured Extended 6 marks

A student closes one eye and holds a pencil at arm's length in front of the open eye, keeping it in focus as they slowly bring it closer to their face.

(a) Name the muscles that control the shape of the lens, and name the structures that connect these muscles to the lens. [2]

(b) Describe how the shape of the lens changes as the pencil moves closer to the eye, and explain how the muscles and structures named in (a) bring about this change. [3]

(c) State how the shape of the lens differs when the student instead focuses on a distant object across the room. [1]

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Worked solution

Part (a): The muscles and structures controlling lens shape

The shape of the lens is controlled by the ciliary muscles, a ring of muscle surrounding the lens. These muscles are attached to the lens by the suspensory ligaments, which run between the ciliary muscles and the edge of the lens.

Part (b): Accommodation for a near object

As the pencil is brought closer to the eye, the lens must refract light more strongly to focus the image on the retina, so it becomes fatter (more rounded/more convex):

  1. The ciliary muscles contract, reducing the diameter of the ring they form around the lens.
  2. This slackens (loosens) the suspensory ligaments, releasing the tension that was pulling on the edges of the lens.
  3. Because the lens is elastic, it naturally springs into a fatter, more curved shape once this tension is released.
  4. The fatter lens refracts light more strongly, bending it enough to focus the close pencil sharply on the retina.

Part (c): Focusing on a distant object

For a distant object, less refraction is needed, so the lens becomes thinner (flatter). Here, the ciliary muscles relax, which pulls the suspensory ligaments tight, and this tension stretches the lens into a thinner, flatter shape.

Final answers

  • (a) Ciliary muscles, connected to the lens by the suspensory ligaments.
  • (b) The lens becomes fatter: the ciliary muscles contract, the suspensory ligaments slacken, and the elastic lens bulges into a more rounded shape that refracts light more strongly.
  • (c) For a distant object, the lens is thinner (flatter) than it is for a near object.