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

The inability of a normal eye to see objects clearly when they are placed closer than 25 cm is due to the physiological limitation known as the near point. The near point is the closest point to the eye at which an object can be brought into focus, and it is generally around 25 cm for a normal, emmetropic (non-presbyopic) eye.

The primary reason for this limitation is the inability of the eye’s lens to adjust its focal length sufficiently to bring close objects into sharp focus. The lens of the eye can change its shape through a process called accommodation, controlled by the ciliary muscles. When the ciliary muscles contract, the lens becomes more convex, increasing its refractive power for focusing on nearby objects.

However, there is a limit to the extent of accommodation. As an object is brought closer to the eye, the ciliary muscles contract to increase the curvature of the lens, but there comes a point where the lens cannot adjust any further. Beyond this point, the light rays from a close object converge too much, and the image is formed behind the retina instead of directly on it. As a result, the image appears blurred.

The near point represents the minimum distance at which the lens can focus light properly on the retina. Beyond the near point, the eye requires additional optical assistance (such as reading glasses) to converge light properly and form a clear image on the retina.

If we say in overall, the inability of a normal eye to see objects clearly when placed closer than 25 cm is due to the limited accommodation range of the eye’s lens, which prevents it from adjusting sufficiently to focus on objects at very close distances.