# Calculating power of lens?

## 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

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Simon Bridge
Homework Helper
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 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?

Simon Bridge