Find the change in focal length

[lha08]

Homework Statement

A nearsighted person has eyeglasses with a -1D prescription. If the near point is at 23 cm without eyeglasses, where is it when they are put on?

Homework Equations

(1/f)=(1/do)+(1/di)

The Attempt at a Solution

I got the answer which is 0.298701298 meters by -1= (1/do)-(1/0.23)...but the problem is that i don't know why it's like that...Any ideas would be extremely helpful! Thanks!

 

[mgb_phys]

You have that equation wrong,
f = the focal length of the lens ( -0.23m)
di is the image distance (which you don't know - but can assume is the same )
do is the object distance

Then you need the formula for a combination of two lenses (ie the eye + glasses) to find the change in focal length.