What is the near-point of the eye-plus-spectacle?

[gungo]

Homework Statement

A hyperopic eye has a near point of 75 cm. If a lens of power .75 d is placed in front of the eye, what is the near point of the eye-plus spectacle?

Relevant Equations

P= 1/p + 1/q

I found the power of the eye itself by doing P= 1/.25 m + 1/ -.75 m. I used negative .75 m because the image would be on the same side of the object. I got a power of 2.67 d. The I added that to the .75 d of the lens. Then I used the same equation and did 3.42= 1/.25 m +1/q to find q, and got -1.72 m. However, the answer is 48 cm.

 

[BvU]

Hi,

Where does the 0.25 come from?

What power is really needed for a near point of 0.75 m ?

 

[gungo]

Hi,

Where does the 0.25 come from?

What power is really needed for a near point of 0.75 m ?

I learned in class that 0.25 m was the near point of a normal eye

 

[BvU]

the near point of a normal eye

So a normal eye has a power of 4 d. But:
This exercise is about a different eye ! Namely, with a power of ...