An optical instrument used for angular magnification has a 25 D objective and a 20 D eyepiece.

The tube length is 25 cm when the eye is least strained. (A) Whether it is a microscope or a tele

scope? (B) What is the angular magnification produced?

Given data

For objective lens-

Power of the objective lens = 25 D

Using lens formula, the focal length of the objective lens becomes-

Power of eyepiece-

Power of the eyepiece = 5 D

The focal length of the eyepiece is given by

(a) The focal length of objective is less than the eyepiece,

Hence the instrument is a microscope

(b)Given data

length of the Tube , l = 25 cm

Distance between image and eye lens, v_e = 25 cm

The length of the tube ,l is given by-

Using lens formula for objective lens-

For the microscope, the angular magnification m is

Hence the angular magnification required is 20.