For the thin convex lens, the radii of curvature are at \( 15 \, \text{cm} \) and \( 30 \, \text{cm} \) respectively. The focal length the lens is \( 20 \, \text{cm} \). The refractive index of the material is:
\( 1.8 \)
\( 1.4 \)
\( 1.5 \)
\( 1.2 \)