How do I find the refractive index of a plano-convex lens (in air) when given the focal length and radius of curvature?

Take a look here:
Focal length - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Focal_length#General_optical_systems)

The formula is :

\frac1f = (n-1) \[\frac{1}{R_1} - \frac{1}{R_2} + \frac{(n-1)d}{nR_1R_2} \]

Since your lens is a planoconvex one, I guess you'll have to take the other 'radius' to be infinite. Let the 'other radius', which is the flat part be R_2

\frac1f = (n-1) \[\frac{1}{R_1} - \frac{1}{\infty} + \frac{(n-1)d}{n R_1\infty} \]

\frac1f = (n-1) \[ \frac{1}{R_1} - 0 + 0 \]

\frac1f =\frac{ (n-1)}{R_1}

I'm not sure for that one though. :o