$A$ convex lens having refractive index $1.6$ has a focal length of $12 \text{ cm}$ when it is in air. The focal length of that lens when placed in water is (refractive index of water = $1.28$) (in $\text{ mm}$)

Explain the first focal point and second focal point for a lens.

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 distance between an object and a screen is $100\, cm$. $A$ lens can produce a real image of the object on the screen for two different positions between the screen and the object. The distance between these two positions is $40\, cm$. If the power of the lens is close to $\left(\frac{N}{100}\right) D$ where $N$ is an integer,the value of $N$ is.......

If the refractive index from air to glass is $\frac{3}{2}$ and that from air to water is $\frac{4}{3}$,then the ratio of focal lengths of a glass lens in water and in air is

The figure given below shows a beam of light converging at point $P.$ When a concave lens of focal length $16 \ cm$ is introduced in the path of the beam at a place $O$ shown by the dotted line such that $OP$ becomes the axis of the lens,the beam converges at a distance $x$ from the lens. The value of $x$ will be equal to.....$cm$.