Index of refraction for converging lens

[vetgirl1990]

Homework Statement

A thin converging lens has radii of curvature of R1=9cm and R2=-11cm. Assume its focal points F1 and F2 are 5cm from the center of the lens. Determine its index of refraction.

Homework Equations

Len's Makers equation: 1/f = (n2 - n1 / n1) (1/R1 - 1/R2)

The Attempt at a Solution

focal length f = 5cm
n1 = air = 1

Plugging in all my known values:
1/5 = (n2 - 1) (1/9 - 1/11)
n2 = 10.9

This value for refractive index doesn't make sense at all... so I'm not sure what I'm doing wrong!

 

[TSny]

Did you take into account that R₂ is negative?

 

[vetgirl1990]

Did you take into account that R₂ is negative?

D'oh! What a ridiculous mistake. Yes, with that taken into account the refractive index is 1.99.

 

[TSny]

OK, good.