The Human Eye and the Colourful World · Lesson 2 of 8
Power of Accommodation
“Your eye changes focus faster than you can say ‘where did I put my glasses?’”
• Explain how ciliary muscles change the curvature and focal length of the eye lens. • Compare the focusing of nearby and distant objects. • Define accommodation, near point, far point and least distance of distinct vision. • Explain why an object held too close appears blurred or causes strain. • Describe cataract and how vision may be restored.
Shift your gaze from a distant tree to a word printed on this page. The word becomes sharp almost immediately, even though the distance between your eye lens and retina has not been rearranged. The eye solves the focusing problem by changing the shape of its own flexible lens. This ability allows one eye to produce clear retinal images for a wide range of object distances.
The eye lens is made of a fibrous, jelly-like material. Ciliary muscles attached around it can alter its curvature to a limited extent. A change in curvature changes the focal length: a flatter, thinner lens bends light less strongly and has a longer focal length, whereas a more curved, thicker lens bends light more strongly and has a shorter focal length.
The ability of the eye lens to adjust its focal length so that both nearby and distant objects can be focused clearly on the retina.
Focusing On Distant And Nearby Objects
When the eye looks at a distant object, the arriving rays are nearly parallel. The ciliary muscles relax and the lens becomes relatively thin. Its focal length increases, providing just enough convergence to bring the rays to the retina. A strongly curved lens would converge these rays too soon, so the relaxed, thinner form is appropriate.
When the eye turns to a nearby object, the rays entering it are more divergent. They require greater converging action. The ciliary muscles contract, increasing the curvature and thickness of the lens. The focal length decreases, and the more powerful lens again brings the rays to the fixed retinal surface. Accommodation changes focal length; it does not move the retina backward or forward.
| Viewing condition | Ciliary muscles | Lens shape | Focal length |
|---|---|---|---|
| Distant object | Relax | Thinner and less curved | Increases |
| Nearby object | Contract | Thicker and more curved | Decreases |
Problem
A learner looks from a nearby notebook to a distant building. What changes occur in the eye?
- 1.The object changes from nearby to distant, so the incoming rays become less divergent and nearly parallel.
- 2.The ciliary muscles relax.
- 3.The eye lens becomes thinner and less curved.
- 4.Its focal length increases until the building is focused on the retina.
Problem
A learner first sees a distant board clearly and then focuses on a pencil held nearby. Explain the complete adjustment without saying that the retina moves.
- 1.The nearby pencil sends more divergent rays into the eye.
- 2.The ciliary muscles contract and increase the curvature of the lens.
- 3.The thicker lens has a shorter focal length and greater converging action.
- 4.The rays are brought to the same fixed retina, producing a sharp image.
Problem
When the distance of an object from a normal eye increases, what happens to the image distance and to the focal length of the eye lens?
- 1.The image must continue to form on the retina, whose distance from the lens is essentially fixed. Therefore, the image distance within the eye remains nearly unchanged.
- 2.As the object moves farther away, the incoming rays become less divergent.
- 3.The eye lens becomes thinner and its focal length increases.
- 4.The crucial distinction is that object distance changes greatly, lens focal length adjusts, but the retinal image distance does not follow the object outward.
Near Point Of The Eye
The eye lens cannot become indefinitely curved. There is a minimum focal length that a healthy eye can produce, so there is also a minimum object distance at which it can focus without strain. For a young adult with normal vision, an object is generally held about 25 cm from the eye for comfortable, distinct viewing.
The minimum distance from the eye at which an object can be seen clearly and comfortably without strain. It is also called the near point and is about 25 cm for a young adult with normal vision.
If a printed page is brought closer than the near point, the rays arriving at the eye are too divergent for the lens to focus even at its greatest normal curvature. The image then tends to form beyond the retina, so the print looks blurred. Continued effort by the ciliary muscles may also produce discomfort. Moving the page away reduces divergence and restores a clear image.
Far Point Of The Eye
The farthest point from the eye at which an object can be seen clearly. For a normal eye, the far point is at infinity.
The normal eye can therefore see clearly over a range extending from approximately 25 cm to infinity. ‘Infinity’ here means that the object is sufficiently distant for the rays reaching the eye to be treated as parallel. The eye does not focus beyond infinity; it simply uses its most relaxed focusing state for very distant objects.
Activity
Purpose: To experience the limit of accommodation and estimate why comfortable reading requires a minimum distance. Setup: Use a page with clear print in good, even light. This activity should be done gently; stop if discomfort occurs.
• Hold the page at a comfortable reading distance and focus on one word. • Move the page slowly towards the eye while keeping the same word in view. • Notice the distance at which the word first becomes persistently blurred or the eye begins to strain. • Move the page away again and observe that clarity returns.
Observation and explanation: The word becomes difficult to focus below a certain distance because the lens has reached the limit of its curvature and cannot reduce its focal length further. The exact measured distance can vary with age and individual vision, but about 25 cm is the standard near point for a young adult with normal vision. Conclusion: Accommodation has a finite range.
Cataract
With age, the crystalline lens in some people can become milky or cloudy. This condition is called cataract. The clouded lens does not transmit and focus light normally, so vision may be partly or completely lost. Cataract is different from myopia or hypermetropia: those are mainly focusing defects, while cataract involves loss of lens transparency.
A condition in which the crystalline lens becomes milky and cloudy, causing partial or complete loss of vision.
Vision can often be restored through cataract surgery, in which the clouded lens is treated by surgical replacement. Spectacles alone cannot make a cloudy lens transparent, so cataract should not be confused with a refractive defect corrected merely by changing the external lens power.
Do not keep an object extremely close to the eye for long periods to test the near point. The activity is an observation of comfortable clarity, not a test of how much strain can be tolerated.
Quiz
What happens when the eye focuses on a nearby object?
What is the near point of a young adult with normal vision?
Why does print become blurred when held much closer than the near point?
Which statement about a normal eye is correct?
Which condition involves the eye lens becoming cloudy?
Practice Problems
- Describe the sequence of changes when attention shifts from a distant mountain to a nearby page. Answer: The ciliary muscles contract, the lens becomes thicker and more curved, its focal length decreases, and the more divergent rays from the page are focused on the retina.
- Why is the image distance inside the eye nearly constant when object distance changes? Answer: The retina occupies a nearly fixed position in the eyeball. Clear focus is maintained mainly by altering lens curvature and focal length, not by moving the retina.
- A page is clear at 30 cm but blurred at 15 cm for a normal young eye. Explain. Answer: At 30 cm the page lies beyond the usual near point and can be focused. At 15 cm the rays are too divergent for the lens to focus after reaching its minimum focal length, so the image is blurred.
- Distinguish the near point from the far point. Answer: The near point is the closest distance for clear, strain-free vision and is about 25 cm for a normal young adult. The far point is the greatest clear-viewing distance and is at infinity for a normal eye.
- Why is cataract not simply corrected by an ordinary concave or convex spectacle lens? Answer: Cataract reduces the transparency of the crystalline lens. A spectacle lens can change convergence but cannot remove cloudiness; restoration generally requires surgical treatment of the clouded lens.
Key Takeaways
• Accommodation is the adjustment of eye-lens focal length for clear vision at different distances. • Relaxed ciliary muscles produce a thinner lens with a longer focal length for distant vision. • Contracted ciliary muscles produce a thicker lens with a shorter focal length for nearby vision. • The near point is the closest distance for comfortable clear vision and is about 25 cm for a normal young adult. • The far point of a normal eye is at infinity. • Objects closer than the near point appear blurred because accommodation has a lower focal-length limit. • Cataract is clouding of the crystalline lens and can be treated surgically.