Four lenses of focal length `+15 cm , +20 cm, +150 cm` and `+250cm` are available for making an astronomical telescope. To produce the largest magnification, the focal length of the eye-piece should be

Options

A. `+15 cm`
B. `+20 cm`
C. `+150 cm`
D. 50 cm

Answer

A

Explanation

The focal lengths of the lenses are
`f_(1)=50cm, f_(2)=20 cm, f_(3)=15 cm and f_(4)=150 cm`
M.P. of a telescope `=(f_(o))/(f_(e))` for normal adjustments
Thus, `M.P. prop (1)/(f_(e))`
Hence for a large magnification, the focal length of the eyepiece should be minimum i.e. the lens having f = 15 cm should be used.