Calculating Power of Tear Layer in Contact Lens Prescription

[AngelLegs]

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

 

[Simon Bridge]

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?

 

[AngelLegs]

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]

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.