The ratio of the refractive index of red light to blue light in air is

$A$ small coin is fixed at the centre of the base of an empty cylindrical steel container having radius $R=1 \,m$ and height $d=4 \,m$. At time $t=0$,the container starts getting filled with water at a flow rate of $Q=0.1 \,m^3/s$ without disturbing the coin. Find the approximate time $t$ in seconds when the coin will first be seen by the observer $O$ from a height of $H=5.75 \,m$ above and $L=1.5 \,m$ radially away from the coin as shown in the figure. (Take refractive index of water,$n=1.33$ or $4/3$)

$A$ student sees the top edge and the bottom centre $C$ of a pool simultaneously from an angle $\theta$ above the horizontal as shown in the figure. The refractive index of water which fills up to the top edge of the pool is $\frac{4}{3}$. If $\frac{h}{x}=\frac{7}{4}$,then $\cos \theta$ is

$A$ ray of light of frequency $5 \times 10^{14} \,Hz$ is passed through a liquid. The wavelength of light measured inside the liquid is found to be $450 \times 10^{-9} \,m$. Calculate the refractive index of the liquid.

$A$ light ray passes through four transparent media with refractive indices $\mu_1, \mu_2, \mu_3$ and $\mu_4$ as shown in the figure. All surfaces are parallel to each other. If the emergent ray $CD$ is parallel to the incident ray $AB$,then:

$A$ magician during a show makes a glass lens with $n = 1.47$ disappear in a trough of liquid. What is the refractive index of the liquid? Could the liquid be water?