Question

in a displacement method ,a convex lens is placed between an object and screen.if the magnification in the two positions r m1 and m2 respectively and the displacement between the two positions is x,then the focl length is

Answer 1

Given :

m1=v/u

m2=u/v

m1-m2=v/u-u/v

=v²-u²/uv

=(v+u) (v-u)/uv-------------(1)

now v-u=x

1/f=1/u+1/v

=v+u/uv

1/f=v+u/uv

from equation1 we get :

m1-m2=x/f

f=x/m1-m2

∴focal length is x/m1-m2