The power of a lens (biconvex) is $1.25 {\mathrm{m}}^{-1}$ in particular medium. Refractive index of the lens is $1.5$ and radii of curvature are $20 \mathrm{cm}$ and $40 \mathrm{cm}$ respectively. The refractive index of surrounding medium:

An object '$O$' is placed at a distance of $100 \mathrm{cm}$ in front of a concave mirror of radius of curvature $200 \mathrm{cm}$ as shown in the figure. The object starts moving towards the mirror at a speed $2 \mathrm{cm} {\mathrm{s}}^{-1}$. The position of the image from the mirror after $10 \mathrm{s}$ will be at _____ $\mathrm{cm}$.

As shown in the figure, a combination of a thin plano-concave lens and a thin plano-convex lens is used to image an object placed at infinity. The radius of curvature of both the lenses is $30 \mathrm{cm}$ and refraction index of the material for both the lenses is $1.75$. Both the lenses are placed at distance of $40 \mathrm{cm}$ from each other. Due to the combination, the image of the object is formed at distance $\mathrm{x}=$ ______ $\mathrm{cm}$, from concave lens.

A convex lens of refractive index $1.5$ and focal length $18 \mathrm{cm}$ in air is immersed in water. The change in focal length of the lens will be_____$\mathrm{cm}$. 

(Given refractive index of water$=\frac{4}{3}$)