A biconvex lens made of material with refractive index n2. The radii of curvatures of its left surface and right surface are R1 and R2. The media on its left and right have refractive indices n1 and n3 respectively. The first and second focal lengths of the lens are respectively f1 and f2.
Assume that R1 = R2, n1 ≠ n3. The ratio, f1/f2, of the two focal lengths is equal to
(a) 1
(b) n1/n3
(c) n3/n1
(d) (n3 - 1)/(n1 - 1)
