Question

Where is the near point of an normal eye when accidentally wear a contact lens with a power of +2.0 diopters?

Answer 1

The near point of an eye with power of +2 dopters, u' = - 50 cm

Given:

Power of a contact lens, P = +2.0 diopters

Solution:

To calculate the near point, we need to find the focal length of the lens which is given by:

Power, P =
`(1)/(f)`

where

f = focal length

Thus

f =
`(1)/(P)`

f =
`(1)/(2)` = + 0.5 m

The near point of the eye is the point distant such that the image formed at this point can be seen clearly by the eye.

Now, by using lens maker formula:

`(1)/(f) = (1)/(u) + (1)/(u')`

where

u = object distance = 25 cm = 0.25 m = near point of a normal eye

u' = image distance

Now,

`(1)/(u') = (1)/(f) - (1)/(u)`

`(1)/(u') = (1)/(0.5) - (1)/(0.25)`

`(1)/(u') = (1)/(f) - (1)/(u)`

Solving the above eqn, we get:

u' = - 0.5 m = - 50 cm