The Human Eye and the Colourful World | Class 10 Science Notes

Power of Accommodation

Accommodation is the eye's ability to adjust the focal length of its crystalline lens to focus on objects at varying distances. The eye lens is made of a fibrous, jelly-like material whose curvature can be changed by the ciliary muscles surrounding it. When these muscles relax, the lens becomes thinner, increasing its focal length, which allows clear vision of distant objects. Conversely, when viewing nearby objects, the ciliary muscles contract, causing the lens to become thicker and its focal length to decrease, enabling clear focus on close objects. However, the focal length cannot be reduced beyond a certain minimum limit. For instance, if you try to read a page held very close to your eyes, the image may blur or cause eye strain. The minimum comfortable viewing distance is about 25 cm for a young adult with normal vision, called the least distance of distinct vision or near point. The farthest distance at which the eye can see objects clearly is called the far point, which is effectively infinity for a normal eye. Thus, a normal eye can see clearly between 25 cm and infinity. Additionally, with aging, the crystalline lens may become cloudy, a condition known as cataract, leading to partial or complete vision loss. Cataract can be treated surgically to restore vision.

🔗 Connection: Understanding accommodation leads to exploring common defects of vision when accommodation fails or is impaired, which is the focus of the next section.