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Lens Power (diopters)

  • Thread starter matt72lsu
  • Start date
  • #1
94
0

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)
 

Answers and Replies

  • #2
1,762
59
By your reasoning the original pair of glasses should have been 4 D (P = 1/0.25 = 4.0). Think about it some more.
 
  • #3
94
0
So the only thing I can come up with is .5/.35 = 1.4 (based on what you said)
 
  • #4
94
0
I'm still not sure what I'm doing wrong here. Help?
 
  • #5
mgb_phys
Science Advisor
Homework Helper
7,774
12
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
 
  • #6
94
0
ok i still don't completely understand, but I come up with 3.1 ???
 
  • #7
129
0
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.
 
  • #8
94
0
Ok cool. Thanks everybody!
 

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