Question

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.

Answer 1

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