What is refraction of light?

$A$ 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 $4/3$)

The refractive index of glass is $1.5$ and that of water is $1.3$. If the speed of light in water is $2.25 \times 10^8 \, m/s$,what is the speed of light in glass?

The ratio of the thickness of plates of two transparent mediums $A$ and $B$ is $6 : 4$. If light takes equal time in passing through them,then the refractive index of $B$ with respect to $A$ will be:

Optical phenomena taking place for a mirror and a lens respectively are . . . . . . and . . . . . . .

The refractive index of water and glass with respect to air is $1.3$ and $1.5$ respectively. Then the refractive index of glass with respect to water is