Why is a normal eye not able to see clearly the objects placed closer than 25 cm?
A normal eye is not able to see clearly the objects placed closer than 25 cm because the focal length of the eye lens cannot be decreased below a certain minimum limit. The eye has the ability to adjust its focal length, which is called accommodation, but this adjustment has a limit.
The minimum distance at which objects can be seen most distinctly without strain is called the least distance of distinct vision or the near point of the eye. For a young adult with normal vision, the near point is about 25 cm.
When objects are placed closer than 25 cm, the eye lens cannot decrease its focal length sufficiently to focus the image clearly on the retina, resulting in a blurred image and strain in the eye.
Explanation
The textbook context clearly states that while the eye lens can adjust its focal length through accommodation, there is a minimum limit to this adjustment. The near point (least distance of distinct vision) for a normal young adult is 25 cm. Objects closer than this distance cannot be focused properly because the eye lens cannot reduce its focal length beyond the minimum limit, causing blurred vision.