Case 1: If we take vernier callipers, its least count  is : dv = du = 0.01cm = 10^-4m

Case 2: If we take meter scale, its least count  is : dv = du = 0.1cm = 10^-3m

v-image distance

u-object distance

f-focal length

then 1/v + 1/u = 1/f

for v=0.6m and u=0.3m , we have (from above formula) f = 0.2m

on differentiating above eqn. for error analysis, we have

 -dv/v² -du/u² =  -df/f²

Case 1(vernier scale):  df = f²( dv/v² +du/u² )

                                                = 0.04*(1/0.36 + 1/0.09)*10^-4m

                                                = 5.56x10^-5 m

                                    focal length limits are : ( 0.2 ± 5.56x10^-5 ) mts

Case 2(metee scale):  df = f²( dv/v² +du/u² )

                                              = 0.04*(1/0.36 + 1/0.09)*10^-3m

                                              = 5.56x10^-4 m

                                      focal length limits are : ( 0.2 ± 5.56x10^-4 ) mts

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Naga Ramesh
IIT Kgp - 2005 batch