NEET Class neet Answered

if focal length of objective lens is 40 cm , then lens-to-image distance v_o of image fromed from an object that is at a distance 200 cm from objective lens is calculated from lens equation

In above equation v = v_o ,  u = -200 cm and f = 40 vm

Image formed by objective lens is object of eyepiece.

Let u_e is the lens-to-object distance of eyepiece. We want lens-to-image distance of eyepiece as D = 25 cm ( least distance of distinct vision ).

Hence we get from lens equation as

where f_e is focal length of eye piece lens .

Hence u_e = -3.45 cm

Distance d between objective lens and eyepiece is

d = | v_o | + | u_e | = ( 50 + 3.45 ) cm = 53.45 cm

( we can consider the round-off distance as 54 cm )