A planoconvex lens ,when silvered at its plane surface is equivalent to a concave mirror of focal length 28cm when its curved sruface si silveredand the plane surface nor silvered ,tis is equivalent to a concave mirror of focal length 10cmthen the refractive index of the material of the lens is:

A planoconvex lens ,when silvered at its plane surface is equivalent t

1.	A planoconvex lens ,when silvered at its plane surface is equivalent to a concave mirror of focal length 28cm when its curved sruface si silveredand the plane surface nor silvered ,tis is equivalent to a concave mirror of focal length 10cmthen the refractive index of the material of the lens is:
Answer» <P>`9//14` `14//9` `17//9` none Solution :When plane surface is silvered `P_(eq)=2P_(1)+P_(m)rArr (1)/(-f_(eq)) =(2)/(f_(1))` `(1)/(-(-28))=(2)/(f_(1)) rArr f_(1)=56cm` `(1)/(f_(1))=(MU-1)((1)/(R_(1))-(1)/(R_(2))) rArr (1)/(56)=((mu-1))/(R) ……(1)` when curved surface is silvered `P_(eq)=2P_(1)+P_(m) rArr (1)/(-f_(eq))=(2)/(f_(1))-(1)/(f_(m))` `(1)/(-(-10))=-(2)/(56)-(1)/(f_(m)) rArr f_(m)=(-280)/(18)` `R=(-280)/(9)....(2)` From (1)&(2) `(1)/(56)=((mu-1)9)/(280)` `(mu-1)=(280)/(56xx9) rArr mu-1=(5)/(9) rArr mu=(14)/(9)`