CBSE Class 12-science Answered

Dear Student

If f is the focal length of lence and 

When the lence is kept in the first position to get hte image

u1, v1 are distances of  object and image from lence respectively

   f= u1v1/(u1+v1)-----(1)

inthe second case u=u1+20, v= v1-20

so f=  (u1+20)(  v1-20) /( u1+20 + v1-20)  =[u1v1-400-20u1+20v1]/ (u1+v1)----(2)

Equate qquations 1&2

==>u1=v1+20 ----(3)

but we have u1+v1=60  ---(4)

From Equations 3,4  ===> u1= 40, v1=20

Equn (1)==> f=  800/ 60 =13.33 cm

Regards

Team

Topperlearning