Question

For a norma 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 eye provides a converging power of about 40 diopters, and the least converging power of the eyelens behind the cornea is about 20 diopters. From this rough data estimate the range of accommodation (i.e., the range of converging power of the eyelens) of a normal eye.

Answer 1

To observe objects at infinity, the eye uses its least converging power
`=40+20=60D`
`therefore` Distance between cornea eye lens and retina
focal length of eye lens `=(100)/(P)=(100)/(60)`
`=(5)/(3)cm.`
To focus and object at the near point
`u=-25cm, v=5//3cm`
`f=?`
`(1)/(f)=-(1)/(u)+(1)/(upsilon) rArr (1)/(f)=(1)/(25)+(3)/(5)=(1+15)/(25)`
`=16//25`
`f=25//16cm`
`"Power "=(100)/(f)=(100)/(25//16)=64D`
`"Power of eye lens "=64-40=24D`
Hence range of accommodation of eye lens is roughly 20 to 24 dioptre.