Question

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

Answer 1

Because the focal length of Normal eye is 25cm.
When any object placed closer than 25cm then the image is FORMED behind the retina. Then object seem blurred.

Answer 2

For a ‘normal eye’ to visualise the objects focussing must have a ‘focal length’ of greater than 25cm, if the eye has to focus particles ‘less than 25 cm’ then the ciliary muscles of the see should change position such as to change its ‘focal length’ in order to visualise objects closer than 25 cm.

Focal length is the “distance between the centre of the lens and its focus object”.