Refractive Index – Lens question!

1. The problem statement, all variables and given/known data

A double concave lens has equal radii of curvature of 15.1 cm. An object placed 14.2 cm from the lens forms a virtual image 5.0 cm from the lens. What is the index of refraction of the lens material?

A. 1.98
B. 1.9
C. 1.84
D. 2.06

2. Relevant equations

1/f = (n – 1 ) (1/R1 – 1/R2 )
1/f = 1/do + 1/di

3. The attempt at a solution

1/f = 1/14.2 + 1/5.0 so i get 1/f = 0.27

1/f = (n – 1 ) (1/R1 – 1/R2 )
0.27 = (n-1) (1/15.1 – (-1/15.1)

this results in an n equal to about 3.03 which is not close to any of the answers, though I’m sure i have the right equations, I’m just not sure what i’m doing wrong?
are these only applicable to convex lens?
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

http://ift.tt/1o5SNst

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