$A$ convex lens:

$A$ convex lens of focal length $f$ is placed somewhere in between an object and a screen. The distance between the object and the screen is $x$. If the numerical value of the magnification produced by the lens is $m$, then the focal length of the lens is

$A$ magnifying lens has a focal length of $10 \, cm$. If the image is formed at a distance of $30 \, cm$ from the lens,what is the magnification?

The focal length of a thin lens made from a material of refractive index $1.5$ is $15 \ cm$. When it is placed in a liquid of refractive index $\frac{4}{3}$,its focal length will be $..........\ cm$.

An object is placed first at infinity and then at $20 \ cm$ from the object-side focal plane of a convex lens. The two images thus formed are $5 \ cm$ apart. The focal length of the lens is $...... \ cm$.

$A$ lens behaves as a converging lens in air but a diverging lens in water,then the refractive index $(\mu)$ of its material is