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

Explanation

Total power = 40 + 20 = 60 dioptre
u = - `infty , f = (1)/(P) = (1)/(60) m, (1)/(v) - (1)/(u) = (1)/(f) " " therefore (1)/(v) - (1)/((-infty)) = 60 " " therefore v = (1)/(60) m = (100)/(60) ` cm
i.e., v = `(5)/(3) ` cm
To focus the object at the near point , u = - 25 cm, v = `(5)/(3) ` cm
`therefore (1)/(f) = (1)/(v) - (1)/(u) = (3)/(5) - (1)/((-25)) = (3)/(5) + (1)/(25) = (15 + 1)/(25) = (16)/(25) " " therefore " power" (1)/(f) = (16)/(25)xx 100 = 64 ` dioptres