$A$ thin convex lens of refractive index $1.5$ has a focal length of $15\, cm$ in air. When the lens is placed in a liquid of refractive index $4/3$,its focal length will be......$cm$.

For a given lens,the magnification was found to be twice as large when the object was $0.15 \ m$ distant from it as when the distance was $0.2 \ m$. The focal length of the lens is.......$m$.

An object is placed at $15 \, cm$ from a convex lens of focal length $20 \, cm$. Which of the following is correct?

$A$ plano-convex lens having a radius of curvature of the first surface $2 \ cm$ exhibits a focal length of $f_1$ in air. Another plano-convex lens with a first surface radius of curvature $3 \ cm$ has a focal length of $f_2$ when it is immersed in a liquid of refractive index $1.2$. If both lenses are made of the same glass of refractive index $1.5$,the ratio of $f_1$ and $f_2$ will be:

The relation between $n_1$ and $n_2$,if the behavior of light rays is as shown in the figure,is:

The focal length of an equi-convex lens is greater than the radius of curvature of any of the surfaces. Then the refractive index of the material of the lens is