Question

A convex lens made up of glass of refractive index $1.5$ is immersed in a medium of refractive index $1.65$. What is the nature of lens in the medium?

Answer 1

from lens maker's formula

$\frac{1}{f}=\left(\frac{{\mu }_2-{\mu }_1}{{\mu }_1}\right)[\frac{1}{R_1}-\frac{1}{R_2}]$

given, ${\mu }_2=1.5$ and ${\mu }_1=1.65$

Since, ${\mu }_2-{\mu }_1=$ -ve

focal length, $f$ will also be negative

Negative focal length means concave (diverging) lens