Question

You are given two converging lenses of focal lengths 125 cm and 5 cm to design a compound microscope. If it is desired to have a magnification of 30, find out the separation between the objective and the eyepiece.

Answer 1

Given $f_{ o }$ = 1.25 cm $f_{ e }$ = -5 cm

Magnification, m= 30,

D= 25 cm

If the object is very close to the principal focus of the objective and the image formed by the objective is very close to eyepiece, then magnifying power of a microscope is given by

m = -L/$f_{ o }$ . D / $f_{ e }$

====> 30 = L/1.25 . 25/5

===> L = 125x30x5/25x100

====> L = 25 x 30/100

===> L = 30/4

===> L = 7.5 cm

This is a required separation between the objective and the eyepiece.