For a normal eye, the far point is at infinity and the near point of distinct vision is about `25 cm` in front of the eye. The cornea of the provides a converging power of about `40 dioptre` and the least converging power of eye lens behind the cornea is about `20 dioptre`. From this rough data, estimate the range of accommodation (i.e., the range of converging power of the eye lens) of a normal eye.

Explanation

To see objects at infinity, the eye uses its least converging power, `P=(40+20)D=60D`. This gives a rough idea of the distance between the retina and cornea - eye lens.
So focal length,
`f=(1)/(P)`
`"or "f=(1)/(60)m=(5)/(3)cm`.
To focus an object at the near point `(u=-25cm)`, on the retina `(v=5//3cm)`, the focal length should be `(1)/(v)-(1)/(u)=(1)/(f)`
`(3)/(5)+(1)/(25)=(1)/(f)`
`(15+1)/(25)=(1)/(f)`
`"or, "f=(25)/(16)cm`
`therefore" Converging power "=(1)/(f)=(1,600)/(25)=64D`
`therefore" Power of the eye lens, "P_(e)=64-40=24D`
`therefore` The range of accommodation of the eye lens is approximately 20 D to 24 D.