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

Options

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

Answer

C

Explanation

Angular magnification of an astronomical refracting telescope is given as
`M=f_(0)/f_(e)`
where, `f_(0)` and `f_(e)` are the focal length of objective and eye-piece, respectively. From the given relation, it is clear that for large magnification either `f_(0)` has to be larger or `f_(e)` has to be small.
Angular resolution of an astronomical refracting telescope is given as
`R=a/(1.22lambda)`
where, a is the diameter of the objective.
Thus, to have large resolution, the diameter of the objective should be large.
Hence, from hte above objective lens should have large focal length `(f_(0))` and large diamter (a).