Q. Two convex lenses of focal lengths 0.03 m and 0.05 m are used to make a telescope. The distance kept between the two in order to obtain an image at infinity is
A) 0.35 cm
B) 0.25 cm
C) 0.175 m
D) 0.15 m
Explanation
(a) When the final image is formed at infinity by a telescope, it is called in normal adjustment and then length of the telescope is given by
$L = f_{0}\ + f_{e}$
Where $f_{0}$ is the focal length of objective lens and $f_{e}$ is the focal length of eye-piece
$\therefore L = \ f_{0} + f_{e} = 0.3 + 0.05 = 0.35\ m$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.