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