Physics · JEE | Ray Optics and Optical Instruments

What is the length of a refracting telescope tube when adjusted for normal adjustment (image at infinity)?

1. A
   \( f_o - f_e \)
2. B
   \( \frac{f_o f_e}{f_o + f_e} \)
3. C
   \( \sqrt{f_o f_e} \)
4. D
   \( f_o + f_e \)