Question

An astronomical refracting telescope is being used by an observer to observe planets in normal adjustment. The focal lengths of the objective and eye piece used in the construction of the telescope are $20\, m$ and $2 \,cm$ respectively. Consider the following statements about the telescope :
(a) The distance between the objective and eye piece is $20.02\, m$
(b) The magnification of the telescope is $(-) 1000$
(c) The image of the planet is erect and diminished
(d) The aperture of eye piece is smaller than that of objective
The correct statements are :

• A. (a), (b) and (c)
• B. (b), (c) and (d)
• C. (c), (d) and (a)
• D. (a), (b) and (d)

Answer 1

Answer: (D)

$ f _0=20\, m =2000 \, cm$
$ f _e=2\, cm$
for Normal adjustment
$\rightarrow \text { M.P. }=-\frac{f_0}{f_e}=\frac{-2000}{2}=-1000$
$\rightarrow$ Distance between both lens $= f _0+ f _e$
$=2000+2$
$=2002\, cm$
$=20.02\, m$
$\rightarrow$ Image is inverted and magnified
$\rightarrow$ Aperture of eye piece is smaller than objective.