Asked by shekhar14351 | 16th Feb, 2010, 09:58: AM

Expert Answer:

Dear Student

Say f,u,v are the focal length , object distance and image ddistance respectively.

 the first case.    u1, v1=30 cm       f  =  uv / (u+v)          

==>  f=30u1/(30+u1)---(1)

2nd case            u2= u1-90 cm,  v2=v1+90=120 cm    

==> f=(u1-90)120/((u1-90+120)


Equate 1,2

==>  30u1=(u1-90)120

==>  u1= 4u1-360

  ==>  u1=  120

f=30X120/(30+120)=24 cm





Answered by  | 16th Feb, 2010, 03:23: PM

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