Biology Chapter: Eye Lens Function

The eye lens is made of a fibrous, jelly-like material that can change curvature.
Ciliary muscles adjust lens curvature, modifying its focal length.
Relaxed muscles make the lens thin for viewing distant objects; curvature increases for nearby objects.
Focal length cannot decrease below a specific limit, leading to blurred vision if objects are too close.
The minimum comfortable distance for clear vision is about 25 cm, known as the least distance of distinct vision or near point.
For a young adult with normal vision, the near point is around 25 cm, while the far point is considered infinity.
Cataract is a condition where the lens becomes cloudy, causing partial or complete vision loss, which can be treated with surgery.

Loss of accommodation power can lead to blurred vision, resulting in three common refractive defects.

Myopia allows clear vision of nearby objects but blurs distant ones.
The far point for myopic individuals is closer than infinity, with images formed in front of the retina.
Causes include excessive lens curvature or elongation of the eyeball.
Corrected using concave lenses that redirect light to the retina.

Hypermetropia provides clear vision for distant objects but blurs nearby ones.
The near point is farther than 25 cm, requiring reading material to be positioned further away.
Light rays from nearby objects focus behind the retina.
Causes include long focal length of the eye lens or a small eyeball.
Corrected with convex lenses to provide additional focusing power.

Presbyopia results from decreased accommodation power due to aging, making nearby objects difficult to see clearly.
The near point gradually recedes, necessitating corrective glasses for comfortable near vision.

Eye donation can significantly benefit visually impaired individuals, with around 35 million people in the developing world affected by blindness.
Corneal blindness affects approximately 4.5 million people, often treatable through corneal transplantation.