The velocity of light in glass,whose refractive index with respect to air is $1.5$,is $2 \times 10^8 \ m/s$. In a certain liquid,the velocity of light is found to be $2.5 \times 10^8 \ m/s$. The refractive index of the liquid with respect to air is:

Shown in the figure is a transparent tank of length $30 \,cm$. $A$ black strip of width $3.8 \,cm$ is stuck on its left wall. When a source of light is kept to the left of it,a shadow of width $7.6 \,cm$ is formed on the right wall. Now,the tank is filled with a liquid of refractive index $n$,and the width of the shadow reduces to $6.4 \,cm$. The value of $n$ is closest to

In the figure shown,$\frac{\sin i}{\sin r}$ is equal to:

$A$ light wave travels through three transparent materials of equal thickness. Rank in order,from the highest to lowest,the indices of refraction $n_1, n_2$ and $n_3$.

The wavelength of light in air and some other medium are respectively ${\lambda_a}$ and ${\lambda_m}$. The refractive index of the medium is:

$A$ ray of light $AO$ in vacuum is incident on a glass slab at an angle of $60^o$ and refracted at an angle of $30^o$ along $OB$ as shown in the figure. The optical path length of the light ray from $A$ to $B$ is