Question

Given that the diameter of eyeball is about 2.3 cm and a normal eye can adjust the focal length of it's eye lens to see objects situated anywhere from 25cm to an infinite distance away from it.
What is the power of the (normal) eye lens, when ciliary muscles are fully relaxed?

Answer 1

A normal eye can see objects from 25cm (near point) till infinity (far point).

So, we take:

Object distance, u = -∞ (- infinity)

Image distance, v = 25 cm

By using lens formula, we get,

1/v - 1/u = 1/f

1/25 - 1/-∞ = 1/f

1/f = 1/25 + 0           [Since, 1/-∞ = 0]

1/f = 1/25

f = 25 cm

For determining the power of the lens, focal length should be in metres.

Therefore, 25/100 = 0.25 m

Power, P = 1/f

P = 1 / 0.25

P = 4 D

Therefore, the power of a normal eye is 4 Dioptres.