Ray Optics and Optical Instruments - Result Question 64

67. An astronomical refracting telescope will have large angular magnification and high angular resolution, when it has an objective lens of [2018]

(a) small focal length and large diameter

(b) large focal length and small diameter

(c) small focal length and small diameter

(d) large focal length and large diameter

Show Answer

Answer:

Correct Answer: 67. (d)

Solution:

  1. (d) For telescope, angular magnification $=\frac{f_0}{f_E}$

So, focal length of objective lens should be large.

Angular resolution $=\frac{D}{1.22 \lambda}$ So, $D$ should be large.

So, objective lens of refracting telescope should have large focal length $(f_0)$ and large diameter $D$ for larger angular magnification.