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, ve = 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.


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