Question

A thin lens of refractive index 1.5, and focal length in air of $20cm$ is placed inside a large container containing two immiscible liquids as shown below. If an object is placed at an infinite distance close to principle axis (common intersection of the two layers), then the distance between images is (in $cm$)

Answer 1

For the upper portion:
$f_1=\frac{{\mu }_1-1}{\frac{{\mu }_2}{{\mu }_1}-1}\left(f_a\right)=\frac{1.5-1}{\frac{1.5}{1.2}-1}\times 20=40cm$
For lower portion,$f_1=\frac{{\mu }_2-1}{\frac{{\mu }_2}{{\mu }_3}-1}\left(f_a\right)=\frac{1.5-1}{\frac{1.5}{2.5}-1}\times 20=-25cm$Object is at infinity, images will form at focal point.
Hence the distance between two images will be $=40+\left(-25\right|=65cm$