The ratio of the focal lengths of a convex lens when kept in air and when it is immersed in a liquid is $1: 2$. If the refractive index of the material of the lens is $1.5$,then the refractive index of the liquid is

What is a lens?

The figure shows a thin lens with centres of curvature $C_1$ and $C_2$. Find its focal length in $cm$. (Take refractive index $\mu = 1.5$)

The formation of a real image using a biconvex lens is shown below:
If the whole setup is immersed in water without disturbing the object and the screen positions,what will one observe on the screen?

The height of the image formed by a converging lens on a screen is $8\,cm$. For the same position of the object and screen,another image of size $12.5\,cm$ is formed on the screen by shifting the lens. The height of the object is...

$A$ convex lens focuses an object $20 \ cm$ from it on a screen placed $5 \ cm$ away from it. $A$ glass plate (refractive index $\mu = \frac{7}{5}$) of thickness $t = 1.4 \ cm$ is inserted between the lens and the screen. What is the new distance of the object from the lens so that its image is again focused on the screen (in $cm$)?