An astronomical telescope arranged for normal adjustment has a magnification of 6. If the length of the telescope is 35cm, then the focal lengths of objective and euye piece respectivley =, are

Options

A. 30cm,5cm
B. 5cm,30cm
C. 40cm,5cm
D. 30cm,6cm

Answer

A

Explanation

In normal adjustment,the object and the final image both are at infinity and the separation between the objective and the eye pieces si `f_(u)+f_(e)`
Thus , `f_(u)+f_(e)=35cm…..(i)`
Also, the magnifying power of the telescope in normal adjustment is `m=-(f_(u))/(f_(e))=-6 ("give")`
`f_(u)=6f_(e) .....(ii)`
On solving Eqs. (i) and (ii) we get
`f_(v)=30cm and f_(e)=5cm`