The absolute refractive index of any medium is always $.....$

What is the angle of incidence for a light ray incident on a glass slab such that it does not undergo refraction (in $^{\circ}$)?

How is the refractive index of a medium related to the speed of light? Obtain an expression for the refractive index of a medium with respect to another in terms of the speed of light in these two media.

What is the effect on the wavelength of light when it travels from a rarer to a denser medium?

Find out,from the table,the medium having the highest optical density. Also,find the medium with the lowest optical density.

Material medium	Refractive index
Air	$1.0003$
Ice	$1.31$
Water	$1.33$
Alcohol	$1.36$
Kerosene	$1.44$
Fused quartz	$1.46$
Turpentine oil	$1.47$
Benzene	$1.50$
Crown glass	$1.52$
Canada Balsam	$1.53$
Rock salt	$1.54$
Carbon disulphide	$1.63$
Dense flint glass	$1.65$
Ruby	$1.71$
Sapphire	$1.77$
Diamond	$2.42$

With respect to air,the refractive indices of ice and rock salt are $1.31$ and $1.54$ respectively. Calculate the refractive index of rock salt with respect to ice.