Question

An astronomical telescope has an angular magnification of magnitude `5` for distant object. The separation between the objective and eyepiece is `36cm` and the final image is formed at infinity. Determine the focal length of objective and eyepiece.
A. `f_0 = 45 and f_e = -9 cm`
B. `f_0 = 50 cm and f_e = 10 cm`
C. `f_0 = 7.2 cm and f_e = 5 cm`
D. `f_0 = 30 cm and f_e = 6 cm`.

Answer 1

Correct Answer - D
Here, Angular magnification of telescope
`= (f_0)/(f_e) = 5`,
`:. f_0 = 5 f_e` …(i)
Now, `f_0 + f_e = 36 cm` ….(ii)
On solving (i) and (ii), we get
`5 f_e + f_e = 36 cm`
`f_e = 6 cm, f_0 = 30 cm`.