Why is a normal eye not able to see clearly the objects placed closer than 25 cm?

A normal eye’s inability to see objects placed closer than 25 cm is primarily due to the physiological limitations of the eye’s focusing mechanism, specifically the ciliary muscles and the elasticity of the eye’s lens. This phenomenon is often referred to as the “near point” or “minimum focusing distance.

1. Lens Elasticity: The eye’s lens is a clear, flexible structure that can change its shape to focus on objects at different distances. This process is called accommodation. However, as we age, the elasticity of the lens decreases. This means that the lens becomes less able to change shape easily to focus on nearby objects.

2. Ciliary Muscles: Accommodation is controlled by the ciliary muscles located around the eye’s lens. When we focus on objects up close, these muscles contract, causing the lens to become thicker and more curved. This increased curvature allows the lens to bend light more effectively, bringing close objects into focus. However, as we age, the ability of these muscles to contract and maintain accommodation decreases, leading to difficulty in focusing on nearby objects.

3. Near Point Limitation: The near point is the closest distance from the eye at which an object can be focused clearly without straining the ciliary muscles excessively. For a typical young adult with normal vision, the near point is approximately 25 cm. This means that attempting to focus on objects placed closer than 25 cm may result in blurry or double vision because the eye cannot effectively accommodate for the increased curvature required to bring the image into sharp focus.

4. Presbyopia: As people age, the ability to accommodate for near objects gradually decreases due to a loss of lens elasticity and reduced ciliary muscle function. This age-related condition is known as presbyopia. It typically becomes noticeable around the age of 40 and progresses over time.

To compensate for presbyopia and the inability to focus on nearby objects, many people require reading glasses or bifocal/progressive lenses to provide the additional focusing power necessary for near vision tasks.

In summary, a normal eye’s inability to see objects clearly when placed closer than 25 cm is due to the limitations of the eye’s lens and ciliary muscles in accommodating for very close distances. This limitation becomes more pronounced with age and is known as presbyopia.