relation between refractive index and density of medium

[Geocentric]

It is said that the density and refractive index of a medium are related by the equation,
(n - 1)/d = constant
where n = refractive index, d = density
Is it true?

[ronthepon]

Surely that can't be true! There are oils less dense than water, yet more optically refractive.

[Tim_Lou]

it might be true for a certain type of materials, but it cannot be true in general.

index of refraction is obtained via experiements. so perhaps your equation is simply a linear fit for some situations.

[UncleAl]

Refractive index is roughly proportional to the square root of the products of medium electrical and magnetic polarizability (e.g., permittivity and permeability). Refractive index increasing with density, mass/volume, is a red herring poorly coupled to increased polarizability of heavy elements.

Sulfur (rhombohedral):
d = 2.07 g/cm^3
nD = 1.957

Bromine:
d = 3.119 g/cm^3
nD = 1.661

Calcium fluoride:
d = 3.180 g/cm^3
nD = 1.434

Benzene:
d = 0.874
nD = 1.501

Hexadeuterobenzene:
d = 0.95 g/cm^3
nD = 1.497

Hexafluorobenzene:
d = 1.612 g/cm^3
nD = 1.332

[Tim_Lou]

yeah uncleAI is right:
the speed of light, this can be derived right out of maxwell's equation:

[Geocentric]

Originally Posted by UncleAl

Refractive index is roughly proportional to the square root of the products of medium electrical and magnetic polarizability (e.g., permittivity and permeability). Refractive index increasing with density, mass/volume, is a red herring poorly coupled to increased polarizability of heavy elements.

Sulfur (rhombohedral):
d = 2.07 g/cm^3
nD = 1.957

Bromine:
d = 3.119 g/cm^3
nD = 1.661

Calcium fluoride:
d = 3.180 g/cm^3
nD = 1.434

Benzene:
d = 0.874
nD = 1.501

Hexadeuterobenzene:
d = 0.95 g/cm^3
nD = 1.497

Hexafluorobenzene:
d = 1.612 g/cm^3
nD = 1.332

That great. Thanks.

[UncleAl]

Substitute fluorine for hydrogen. Density increases, refractive index decreases. Fluorine is heavy but not polarizable.

nonane, C9H20
MW = 128.26
d = 0.718 9/cm^3
nD = 1.405

perfluorononane, C9F20
MW = 488.06
d = 1.799 g/cm^3
nD = 1.267

[Savant]

I agree with UncleA1 -- sort of. There is a relationship between density and refractive index. Clearly the actual refractive index of bromine at near vacuum is less than liquid bromine. The published (standardized) refractive index of bromine is a refractive index measured at standard conditions thus eliminating density variance. The same can be said of any other material and comparison of refractive indexes of varying materials are also taken at standard conditions that do not permit an easy comparison of density effects.

[Savant]

Out of curiosity, where did GEOCENTRIC obtain this formula?

[ZAP-st]

It is clear that refractive index (RI) is roughly proportional to the square root of the products of medium electrical and magnetic polarizability (e.g., permittivity and permeability). However, the permittivity is essentially the dielectric constant (Kd) of a material. The dielectric constant for water is ~88 while that for cyclohexane is ~. 2.
So I am trying to understand, for example, why the RI of cyclohexane (1.46) is greater that that of water (1.33) at 589nm.
For the permeability the relationship is

The relative permeability Km is equal to magnetic permeability (mu) divided by the, permeability of free space (muo)

This is related to the and magnetic susceptibility Xm by

Km = to 1- Xm

For both water and cyclohexane: Xm is very small making km essentially one for both fluids.

Can someone explain why the RI of cyclohexane is greater that that of water even though the dielectric constant is much smaller? What have I overlooked in the relationship between permittivity, permeability and refractive index?