# Lens Power (diopters)

matt72lsu

## Homework Statement

A middle-aged man starts to wear eyeglasses with lenses of 2.0 D that allow him to read a book held as close as 25 cm. Several years later, he finds that he must hold a book no closer than 35 cm to read it clearly with the same glasses, so he gets new glasses.

What is the power of the new lenses?

## Homework Equations

P = 1/f (f in meters)

## The Attempt at a Solution

P = 1/.35 = 2.9 (it was wrong)

skeptic2
By your reasoning the original pair of glasses should have been 4 D (P = 1/0.25 = 4.0). Think about it some more.

matt72lsu
So the only thing I can come up with is .5/.35 = 1.4 (based on what you said)

matt72lsu
I'm still not sure what I'm doing wrong here. Help?

Homework Helper
In diptres the focal length of the eye and the glasses add.

Currently, eyeD + 2D = 1/0.35
Wants, eyeD + xD = 1/0.25
You are trying to find x

matt72lsu
ok i still don't completely understand, but I come up with 3.1 ???

AtticusFinch
That's what I get as well but here's how I did it.

Several years later an object placed 35 cm away from the lens focuses on his retina. Using the thin lens equation I found the retina's distance from the lens. Then I said that a new lens will focus an object at 25 cm at the retina's distance and just used the thin lens equation again.

matt72lsu
Ok cool. Thanks everybody!