Two identical glass `(mu_(g)=3//2)` equiconvex lenses of focal length f are kept in contact. The space between the two lenses is filled with water `(mu_(w)=4//3)` . The focal length of the combination is

Options

A. `4f//3`
B. `3f//4`
C. `f//3`
D. `f`

Answer

B

Explanation

`f_(1)=f_(3)=(R)/(2((3)/(2)-1))=R=f` (given)
`f_(2)=(-R)/(2((4)/(3)-1))=(-3)/(2)R=-(3)/(2)f`
`(1)/(f_(eq.))=(1)/(f_(1))+(1)/(f_(2))+(1)/(f_(3))=(1)/(f)+(-(2)/(3f))+(1)/(f)`
`implies(1)/(f_(eq))=(4)/(3f)impliesf_(eq)=(3f)/(4)`