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CBSE Class 12-science Answered

Dear sir, when we use the formula m=D/u instead of using m=1+D/f for the magnification of a simple microscope please explain in detail.
Asked by modi72879 | 23 Feb, 2018, 09:50: PM
answered-by-expert Expert Answer
Both formulas are one and the same.
Magnification M is obtained from the formula, M = v / u; where v is the image-to-lens distance and u is object-to-lens system.
Here D is the normal distance of least vision of human eye and microscope is desigened to produce the magnified image at D.
 
Hence we get M = ( D / u ).
 
if we consider the lens formula and do liitle simplification we will get the other formula as follows
begin mathsize 12px style 1 over v minus 1 over u space equals space 1 over f a p p l y i n g space s i g n space c o n v e n t i o n comma space v space equals space minus D space a n d space u space i s space n e g a t i v e comma space w e space g e t space fraction numerator 1 over denominator negative D end fraction plus 1 over u equals 1 over f space M u l t i p l y space t h e space a b o v e space r e l a t i o n space b y space D space a n d space u s e space t h e space f o r m u l a space M space equals space D over u comma space w e space g e t space M space equals space 1 plus D over f end style
 
if we know the Object-to-lens distance u and assuming the normal distance of least vision D, we get magnification from M = ( D/u ) . Here we need not know the focal length of the lens.
 
if we know the focal length of lens f and assuming the normal distance of least vision D, we get magnification from M = 1 + ( D / f ) . Here we need not know the object-to-lens distance u.
 
Answered by Thiyagarajan K | 24 Feb, 2018, 02:14: PM
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