# Calculating optical properties of an arbitrary gas?

1. Apr 28, 2015

### Artlav

Hello.

Is there a way to calculate properties of a gas, given only it's composition?

I.e. let's say i want to know what would a planet look like with an atmosphere of a mix of 50% gaseous sulphur and 50% gaseous mercury, at 500*C and surface pressure of half that of Earth.
How would i go about calculating that just from basic laws?

I kind of know where to start regarding the pressure, density and so on, but i have no idea how to get the refractive index and light absorption by wavelength, for example.

Is it even possible?
If so, what should i look for?

2. Apr 28, 2015

### Paul Eccles

These properties (light absorption spectra ) can be experimentally verified, measured, however it is too complicated to compute from pure physical theories. The theories pertaining to light absorption and so on are quantum physics, which don't permit easy computation - anything beyond a helium atoms is to complex to work out,analytically, only approximately. So we are reduced at this level to empirical study in many respects.

3. Apr 29, 2015

### Artlav

Hm, interesting.

Two directions then.

First, how do you compute it for a helium atom, even approximately?
I'm looking for key words.

Second, is there enough empirical data to calculate a refractive index for common gasses at given conditions?
I found data sets online (i.e. http://refractiveindex.info ), but they only show it at or near sea level pressure and temperature.

4. Apr 29, 2015

### Paul Eccles

The Refractive index can be inferred from electromagnetic theory, and density of material. I can tell you that Helium has the record lowest refractive index of all the elements. As for spectral absorption which is more important, that may be calculated with perturbation theory in Quantum Physics. I don't know if we can accurately predict much beyond the spectrum of Hydrogen. We can explain a lot of phenomena. Maybe we can, my Quantum understanding is quite limited.

I can also tell you Gold is gold coloured because of relativity http://en.wikipedia.org/wiki/Relativistic_quantum_chemistry#Color_of_gold_and_caesium

You go look on Physical chemistry charts for the spectral absorption.

Gases in general have very low refractive indices, however the refractive index varies approximately linearly with pressure. (n-1 is proportional to pressure) Furthermore, there are many complications. http://en.wikipedia.org/wiki/Refractive_index