The magnifying power of a microscope with an objective of `5 mm` focal length is 400. The length of its tube is `20cm.` Then the focal length of the eye`-` piece is

Options

A. 200 cm
B. 160 cm
C. 2.5 cm
D. 0.1 cm

Answer

C

Explanation

(c) We assume that final image is formed at infinity, and
`M_(prop)=((L_(oo)-f_(o)-f_(e))cdotD)/(f_(o)f_(e))=(LD)/(f_(o)f_(e))`
or `400=(20xx25)/(0.5xxf_(e))`
`f_(e)=2.5cm`