We wish to make a microscope with the help of two positive lenses both with a focal length of 20 mm each and the object is positioned 25 ~mm from the objective lens. How far apart the lenses should be so that the final image is formed at infinity?

Step-by-step solution

To obtain final image at infinity, object which is the image formed by objective should be at focal distance of eye-piece. By lens formula (for objective) aligned & 1 v₀ - 1 u₀ = 1 f₀ or, & 1 v₀ - 1 -25 = 1 20 & 1 v₀ = 1 20 - 1 25 = 5-4 100 = 1 100 ~mm & v₀=100 ~mm aligned Therefore the distance between the lenses =v₀+f_e=100 ~mm +20 ~mm =120 ~mm