Physics · JEE | Ray Optics and Optical Instruments

What is the expression for the lens maker's formula for a thin lens of refractive index relative to medium, \( n_{21} = n_2/n_1 \), and radii of curvature \( R_1 \) and \( R_2 \)?

1. A
   \( \frac{1}{f} = (n_{21} - 1) (\frac{1}{R_1} + \frac{1}{R_2}) \)
2. B
   \( \frac{1}{f} = (n_{21} + 1) (\frac{1}{R_1} - \frac{1}{R_2}) \)
3. C
   \( \frac{1}{f} = (\frac{n_2}{n_1} - 1) (\frac{1}{R_2} - \frac{1}{R_1}) \)
4. D
   \( \frac{1}{f} = (n_{21} - 1) (\frac{1}{R_1} - \frac{1}{R_2}) \)