# Find the change in focal length

#### lha08

1. 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?

2. Homework Equations
(1/f)=(1/do)+(1/di)

3. 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!!

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#### mgb_phys

Science Advisor
Homework Helper
Re: lenses

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.

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