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Calculating power of lens?

  • Thread starter AngelLegs
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  • #1
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Homework Statement


The contact lens prescribed for a mildly farsighted person is 1.25 D, and the person has a near point of 29.0 cm. What is the power in D of the tear layer between the cornea and the lens if the correction is ideal, taking the tear layer into account? Assume that a normal human has a near point of 25.0 cm.

Homework Equations


P = 1/do + 1/di = 1/f
 

Answers and Replies

  • #2
Simon Bridge
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The tear+contact must make a compound lens with the power needed to correct eyesight with the stated near-point.
How do the powers of lenses combine?
 
  • #3
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The tear+contact must make a compound lens with the power needed to correct eyesight with the stated near-point.
How do the powers of lenses combine?
The two lenses would bee a double convex (in the eye) and the contact would be a convex, right?
But I'm confused about how to set up the problem.
One would be negative and one would be positive, right?
 
  • #4
Simon Bridge
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Go back to basics:
When you are given the nearpoint, and you want to know what power of lense will correct that eyesight - how do you work that out?
When you have two lenses one after the other with P1 and P1 ... how do you find the overall power P of the combined lens?

If you do not answer questions I cannot help you.
Note: the shape of the lens is irrelevant.
 

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