Contact lenses, focal length and uncorrected far point distance

  • Thread starter danok
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  • #1
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1. A nearsighted person wears contacts with a focal length of -7.6cm. If this person's far point distance with her glasses is 8.7m, what is her uncorrected far point distance?



2. 1/F=1/do+1/di



3. 1/-7.6=1/do+1/870
=-7.534


I am completely lost here...please help. Thanks!
 

Answers and Replies

  • #2
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So it's not just me then...
 
  • #3
mgb_phys
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The image distance is constant in both cases (the length of the eyeball)
You have an unknown focal length lens (the eye) and a combintion of the same unknown lens and a known lens with a known object distance.

So write the equations for object, image distance and focal length for both cases.
 

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