When a beam of white light is allowed to pass through convex lens parallel to principal axis, the different colours of light converge at different point on the principle axis after refraction. This is called :

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$=\frac{4}{3}$)

The light rays from an object have been reflected towards an observer from a standard flat mirror, the image observed by the observer are :-

A. Real

B. Erect

C. Smaller in size than object

D. Laterally inverted

Choose the most appropriate answer from the options given below:

An object is placed on the principal axis of convex lens of focal length $10 \mathrm{cm}$ as shown. A plane mirror is placed on the other side of lens at a distance of $20 \mathrm{cm}$. The image produced by the plane mirror is $5 \mathrm{cm}$ inside the mirror. The distance of the object from the lens is ____ $\mathrm{cm}$.

In a medium the speed of light wave decreases to $0.2$ times to its speed in free space. The ratio of relative permittivity to the refractive index of the medium is $x:1$. The value of $x$ is ______.

(Given speed of light in free space $=3\times {10}^8 \mathrm{m} {\mathrm{s}}^{-1}$ and for the given medium ${\mu }_r=1$)