(a) For the telescope described what is the separation between the objective lens and eye piece ?
(b) If this telescope is used to view a `100 m` tall tower `3 km` away, what is the height of the image of the tower formed by the objective lens ?
(c ) What is the height of the final image of the tower if it is formed at 25 cm` ?

Explanation

(a) Here, in normal adjustment,
Separation between objective lens and eye lens `= f_0 + f_e = 140 + 5 = 145 cm`
(b) Angle subtended by tower `100 m` tall at `3 km`
`prop = (100)/(3 xx 1000) = (1)/(30) radian`
If `h` is the height of image formed by the objective, then `prop = (h)/(f_0)=(h)/(140)`
`:. (h)/(140) = (1)/(30)` or `h = (140)/(30) cm = 4.7 cm`
( c) Magnification of final image `= 4.7 xx 6 = 28.2 cm`.