μr is its relative permeability. In non-magnetic media with , Thus refractive index in a non-magnetic media is the ratio of the vacuum wave impedance to the wave impedance of the medium. The reflectivity between two media can thus be expressed both by the wave impedances and the refractive indices as Density In general, the refractive index of a glass increases with its density. However, there does not exist an overall linear relationship between the refractive index and the density for all silicate and borosilicate glasses. A relatively high refractive index and low density can be obtained with glasses containing light metal oxides such as Li2O and MgO, while the opposite trend is observed with glasses containing PbO and BaO as seen in the diagram at the right. Many oils (such as olive oil) and ethanol are examples of liquids that are more refractive, but less dense, than water, contrary to the general correlation between density and refractive index. For air, n − 1 is proportional to the density of the gas as long as the chemical composition does not change. This means that it is also proportional to the pressure and inversely proportional to the temperature for ideal gases. Group index Sometimes, a "group velocity refractive index", usually called the group index is defined: where vg is the group velocity. This value should not be confused with n, which is always defined with respect to the phase velocity. When the dispersion is small, the group velocity can be linked to the phase velocity by the relation where λ is the wavelength in the medium. In this case the group index can thus be written in terms of the wavelength dependence of the refractive index as When the refractive index of a medium is known as a function of the vacuum wavelength (instead of the wavelength in the medium), the corresponding expressions for the group velocity and index are (for all values of dispersion) where λ0 is the wavelength in vacuum. Other relations As shown in the Fizeau experiment, when light is transmitted through a moving medium, its speed relative to an observer traveling with speed v in the same direction as the light is: The refractive index of a substance can be related to its polarizability with the Lorentz–Lorenz equation or to the molar refractivities of its constituents by the Gladstone–Dale relation. Refractivity In atmospheric applications, refractivity is defined