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)

large focal length and large diameter

Answer 2

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 $D$.