Lesson 9 of 10 / Homeostasis and response
Accommodation: focusing near and far
How does the same eye focus a book and a distant sign?
I can explain accommodation using ciliary muscles, suspensory ligaments and lens shape.
I can explain eye functions and near/distant focusing.Syllabus
Syllabus K325, Topic 8(m). Explain eye functions and near/distant focusing.
About 5 min
Make a guess. You are not marked.
You look up from your phone to a distant sign. What happens to the lens in your eye?
Show the answer
It is pulled thinner and less convex.
Ciliary muscles relax, suspensory ligaments tighten, and the lens flattens to focus distant light.
The key idea
For a near object, ciliary muscles contract, suspensory ligaments slacken and the lens becomes more convex, refracting light more strongly.
- Ciliary muscles
- Contract
- Suspensory ligaments
- Slacken
- Lens
- Thicker, more convex
Near light rays need stronger refraction. Reduced ligament tension lets the elastic lens become more convex.
Purple: ciliary muscles. Brown: ligaments. Blue: lens. Gold: representative light rays. This isolates lens adjustment; corneal refraction is omitted from the ray model, and distances are schematic.
Explanation
A clear image forms when light from an object is focused on the retina. The cornea supplies much of the eye's refraction, but its curvature is not adjusted for ordinary near and distant focusing. The lens changes shape to fine-tune the focus. This is accommodation.
Light from a near object reaches the eye as more divergent rays, so stronger refraction is needed. Ciliary muscles contract, reducing tension in the suspensory ligaments. The elastic lens becomes thicker and more convex, increasing its refractive power.
For a distant object, incoming rays are nearly parallel and less additional refraction is needed. Ciliary muscles relax, suspensory ligaments become taut and the lens is pulled thinner and less convex. Light is then focused on the retina.
The image on the retina is real and inverted. Accommodation keeps that image focused by changing lens shape, not by moving the retina. Keep these lens adjustments separate from pupil changes, which regulate the amount of light entering the eye.
Step by step
- 1
Choose near or distant
Near objects require stronger refraction than distant objects.
- 2
Trace the mechanical chain
Ciliary muscle state changes ligament tension, which changes lens shape.
- 3
Explain the optical result
Link a more or less convex lens to refraction and a focused retinal image.
Worked example
Looking up from a book
Explain the changes when the eye shifts from a near book to a distant sign.
One way to explain it
Ciliary muscles relax, suspensory ligaments become taut and the lens becomes thinner and less convex. Its refractive power decreases, allowing the distant object to be focused on the retina.
Why this answer works
- This is a move from near to distant.
- Do not reverse muscle state and ligament tension.
- Finish with the change in refraction and focus.
Watch out for this
A student says: "Contracting ciliary muscles pull the lens thinner for near vision." What is wrong with this?
Show the answer
Contraction reduces tension in the suspensory ligaments. The lens becomes more convex for near vision.
Check your understanding
Which combination focuses a near object?
Show the answer
Ciliary muscles contracted; ligaments slack; lens more convex
The rounder lens refracts the divergent rays from a near object more strongly.
Exam-style questions
Check your understanding
A person looks up from a book to a distant tree. Which change happens in the eye?
Show the answer
Ciliary muscles relax, suspensory ligaments tighten, lens becomes thinner
For a distant object the lens must be thinner, so the ciliary muscles relax and the ligaments pull on the lens.
Quick recall
How does the same eye focus a book and a distant sign?