Answer to Question #94302 in Optics for Sridhar

Question

In an astronomical telescope in normal adjustment a straight black line of length L is drawn on inside part of objective lens. The eye-piece forms a real image of this line. The length of this image is I. The magnification of the telescope is ________
Andeer L/I

Answer 1

The magnification of the telescope is given by formula

$m=\frac {f_o}{f_e}= (1)$

where fo is objective focal distance, fe is the eye-piece focal distance

Mirror formula

$\frac {1}{f}= \frac {1}{v} + \frac {1}{u} (2)$

We can rewrite formula (1) as

$m=\frac {v}{u}=\frac {f}{f+u} (3)$

Image distance is equal to

$u=-(f_o+f_e) (4)$

Using (2),(3),(4) we got

$m=\frac {L}{I}$

Answer

$\frac {L}{I}$