$(a)$ What is meant by the least distance of distinct vision?
$(b)$ How does the thickness of the eye lens change when we shift our focus from a distant tree to reading a book?

(N/A) The least distance of distinct vision is the minimum distance at which an object can be placed so that a normal eye can see it clearly without any strain. For a normal adult eye,this distance is approximately $25 \ cm$.
$(b)$ When we shift our focus from a distant tree to reading a book,the ciliary muscles contract. This contraction increases the curvature of the eye lens,making it thicker. This increase in thickness increases the converging power of the lens,allowing the image of the nearby object (the book) to be focused clearly on the retina.