Finding the optical power of a contact lens

[ilovejava]

Homework Statement

A farsighted man uses eyeglasses with a refractive power of 2.05 diopters. Wearing the glasses 0.020 m from his eyes, he is able to read books held no closer than 0.265 m from his eyes. He would like a prescription for contact lenses to serve the same purpose. What is the correct contact lens prescription, in diopters?

Homework Equations

1/d_o+1/d_i=1/f

The Attempt at a Solution

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The image formed when wearing eyeglasses is 0.265-0.020 = 0.245 and this has to be negative because the man is using convex lenses, so d/_i = -0.245 with eyeglasses. Next, I found the the object distance when wearing eyeglasses by using the formula 1/d_o+1/d_i=1/f and it turned out to be 0.163 the fact that it's positive makes sense because the object is placed in front of the lens. Now because the questions asks to find the power of the contact lenses the distance of the object when using contacts increases by 0.020m so d_o with lenses equals 0.183m. Finally I plugged everything into 1/d_o+1/d_i=1/f and got +1.38 diopters.

 

[andrevdh]

The 0.265 m distance is to the book, which is the object, not the image (for the glasses anyway)!
So what you are calculating for the glasses is the image distance produced by the glasses, which I get a different answer for, so please show your steps to that value so that we are on the same page.