The radii of curvature for a thin convex lens are \( 10 \, \text{cm} \) and \( 15 \, \text{cm} \) respectively. The focal length of the lens is \( 12 \, \text{cm} \). The refractive index of the lens material is:
\( 1.2 \)
\( 1.4 \)
\( 1.5 \)
\( 1.8 \)