A biconvex lens in a liquid

A thin lens has refractive index 1.60 and radii R₁ = +0.24 m and R₂ = −0.24 m. It is immersed in a liquid of refractive index 1.20. Light travels left to right.

Find its focal length in the liquid.

Organize the givens & assumptions

Lens index: 1.60
Liquid index: 1.20
R₁ = +0.24 m; R₂ = −0.24 m
Light travels from left to right

Paraxial thin lens; same liquid on both sides. Each radius is positive when its centre of curvature is to the right of that surface.

A $+0.20\ \mathrm m$
B $+0.30\ \mathrm m$
C $-0.36\ \mathrm m$
D $+0.36\ \mathrm m$

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