Question

An astronomical refracting telescope will have large angular magnification and high angular resolution, when it has an objective lens of

• A. small focal length and small diameter
• B. small focal length and large diameter
• C. large focal length and large diameter
• D. large focal length and small diameter

Answer 1

Answer: (C)

For telescope, angular magnification $=\frac{f_0}{f_E}$
So, focal length of objective lens should be large.
Angular resolution $=\frac{D}{1.22\lambda }$ should be large.
So, objective should have large focal length $\left(f_0\right)$ and large diameter $D$.