A biconvex lens made of material with refractive index `n_(2)`. The radii of curvatures of its left surface and right surface are `R_(1)` and `R_(2)`. The media on its left and right have refreactive indices `n_(1)` and `n_(3)` respectively. The first and second focal lengths of the lens are respectively `f_(1)` and `f_(2)`.
Assume that `R_(1) = R_(2), n_(1) ne n_(3)`. The ratio, `f_(1)//f_(2)`, of the two focal lengths is equal to
A. `1`
B. `n_(1)//n_(3)`
C. `n_(3)//n_(1)`
D. `(n_(3) - 1)//(n_(1) - 1)`
