Question

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

Answer 1

A normal eye is unable to clearly see the objects placed closer than 25 cm because the ciliary muscles of eyes are unable to contract beyond a certain limit.
If the object is placed at a distance less than 25 cm from the eye, then the object appears blurred and produces strain in the eyes.

Answer 2

When the eye is focused on a closer object, the ciliary muscles are strained and the focal length of the eye-lens decreases. Because of some reason, the eye is not able to decrease the focal length beyond a limit, hence a normal eye is not able to see objects closer than 25 cm.