Solution:

By Lens-maker's formula, \( \frac{1}{f} = (\mu - 1) \left(\frac{1}{R_1} - \frac{1}{R_2}\right) \). Substituting \( f = 12 \text{ cm} \), \( R_1 = 15 \text{ cm} \), and \( R_2 = -10 \text{ cm} \), we get \( \frac{1}{12} = (\mu - 1)\left(\frac{1}{15} + \frac{1}{10}\right) = (\mu - 1)\frac{1}{6} ⇒ \mu - 1 = 0.5 ⇒ \mu = 1.5 \).