Question

how does the thickness of the eye lens change when we shift looking from a distant tree to read a book.?

Answer 1

These are the ciliary muscles which control the persistence of vision 

like when we go from light to a dark room our pupil expands 

All these activities controlled by ciliary muscles 

Answer 2

Answer: The thickness of the eye lens increases when we shift looking from a distance tree to reading a book.

Explanation:  When the  muscles are relaxed, the lens becomes thin. Thus, its focal length  increases. This enables us to see distant objects clearly. When you are  looking at objects closer to the eye, the ciliary muscles contract. This  increases the curvature of the eye lens. The eye lens then becomes thicker. ( From NCERT)