- The eye lens is composed of a fibrous, jelly-like material.
- Its curvature/focal length can be modified to some extent by the ciliary muscles.
- The change in the curvature of the eye lens can thus change its focal length.
When you are watching a Far-Off Objects
- The light rays of far off objects come parallel and converges near the eye.
- The Cilliary Muscles is at rekaxed state when looking at a faroff objects
- When the muscles are relaxed, the lens becomes thin. Thus, its focal length
increases. This enables us to see distant objects clearly.
When you Are watching near objects
- When you are looking at objects closer to the eye, the ciliary muscles contract.
- This increases the curvature of the eye lens.
- The eye lens then becomes thicker. Consequently, the focal length of the eye lens decreases.
- This enables us to see nearby objects clearly.
The ability of the eye lens to adjust its focal length is called accommodation.
- However, the focal length of the eye lens cannot be decreased below a certain minimum limit.
- Try to read a printed page by holding it very close to your eyes.
- You may see the image being blurred or feel strain in the eye.
- To see an object comfortably and distinctly, you must hold it at about 25 cm from the eyes.
- The minimum distance, at which objects can be seen most distinctly without strain, is called the least distance of distinct vision.
- It is also called the near point of the eye.
- For a young adult with normal vision, the near point is about 25 cm.
- The farthest point upto which the eye can see objects clearly is called the far point of the eye. It is infinity for a normal eye.
- Cataract – Sometimes, the crystalline lens of people at old age becomes milky and cloudy.
- This condition is called cataract.
- This causes partial or complete loss of vision
DEFECTS OF VISION AND THEIR CORRECTION
- Sometimes, the eye may gradually lose its power of accommodation.
- In such conditions, the person cannot see the objects distinctly and comfortably.
- The vision becomes blurred due to the refractive defects of the eye.
(i) myopia or near-sightedness,
(ii) Hypermetropia or far – sightedness, and
These defects can be corrected by the use of suitable spherical lenses
- Myopia is also known as near-sightedness.
- A person with myopia can see nearby objects clearly but cannot see distant objects distinctly.
- A person with this defect has the far point nearer than infinity.
- Such a person may see clearly upto a distance of a few metres.
- In a myopic eye, the image of a distant object is formed in front of the retina and not at the retina itself.
- This defect may arise due to
(i) excessive curvature of the eye lens, or (ii) elongation of the eyeball.
- This defect can be corrected by using a concave lens of suitable power.
- A concave lens of suitable power will bring the image back on to the retina and thus the defect
- Hypermetropia is also known as farsightedness.
- A person with hypermetropia can see distant objects clearly but cannot see nearby objects distinctly.
- The near point, for the person, is farther away from the normal near point (25 cm).
- Such a person has to keep a reading material much beyond 25 cm from the eye for comfortable reading.
- This is because the light rays from a close by object are focused at a point behind the retina as shown in This defect arises either because
(i) the focal length of the eye lens is too long, or (ii) the eyeball has become too small.
- This defect can be corrected by using a convex lens of appropriate power.
- Eye-glasses with converging lenses provide the additional focusing power required for forming the image on the retina.
- The power of accommodation of the eye usually decreases with ageing.
- For most people, the near point gradually recedes away.
- They find it difficult to see nearby objects comfortably and distinctly without corrective eye-glasses.
- This defect is called Presbyopia.
- It arises due to the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens.
- Sometimes, a person may suffer from both myopia and hypermetropia.
- Such people often require bifocal lenses.
- A common type of bi-focal lenses consists of both concave and convex lenses.
- The upper portion consists of a concave lens.
- It facilitates distant vision.
- The lower part is a convex lens.
- It facilitates near vision.
- These days, it is possible to correct the refractive defects with contact lenses or through surgical interventions