A hydraulic cylinder type setup. At cylinder end is a detector. On the end of the piston is a precise frequency laser emitter. A pure gas is in the piston. As you drive the piston to compress the gas, does it cause changes in what the detector sees? Omitting velocity. Samples are taken at different pressures with the piston stopped so there is no movement. There is no major state change of the gas. You do not pressure it to a liquid state for instance. Ignore reflections. As if the emitter and detector were in an endless expanse of this gas.
The density of the gas is increasing. But the distance is shortened. Is there a difference detected due to pressure differences?
If the emitter was wide band of frequencies. Would pressure change attenuation of frequencies?
It seems to me that if the distance of emitter and detector was constant, there may be differences at pressures. But that they are moving closer as pressure increases, is that cancelled out? Detector moves closer by X amount. Pressure increases by Y amount. Does energy detected remain the same because of the link?
Pressure certainly does affect the absorption of a gas. I once built a device to measure pressure in transparent bottles simply by measuring the transmission.
Of course, it won’t be a big change at all wavelengths – you want to choose one where the absorption is changing significantly, and that might not be in the visible. My device used infrared light.
Refractive index can change, too, as am77494 points out, but unless you have some useful shape on your container (like it’s in the shape of a prism, so you can watch a beam deflect, or it’s like a lens, so you can watch the focal length change) it’s easier to look at changes in transmission.
This depends on the wavelength of the light and the gas you’re using … 525nm light through Hydrogen gas is unaffected at any pressure … your detector will receive all the energy at 525 nm …
But in your case, let’s go ahead and use 103 nm light, because these photons are the exact energy to bump the hydrogen’s electron up from it’s lowest energy state (n = 1) to it’s third energy state (n = 3) … if we’re keeping the temperature the same (see below) … then the hydrogen can’t hold this added energy and quickly releases it again … and some of it will be at 103 nm for our electron’s 3 -> 1 transition … but we can also have a 3 -> 2 transition and the 2 -> 1 transition which are different energies and thus different wavelengths, 656 nm and 122 nm respectively … the detector will receive all the energy put in except the wavelengths will be 103 nm, 122 nm and 656 nm … increasing the pressure only increases the number of hydrogen atoms and so increases the number of transitions … but in the end all the energy passes through if temperature is held constant …
If we don’t control the temperature, then it will go up … the higher pressure will allow more energy to remain with the hydrogen atoms as temperature so our detector will receive that much less energy … input energy must equal output energy plus energy associated with temperature gain … plus whatever energy is needed for the myriad of other processes that could occur, funny things start happening at the quantum level …
Follow-up question to the OP … can this even be done? … I’ve been looking for data to this exact set-up using atmospheric gases of varying concentrations and have been coming up empty … my google-fu ain’t great so a little help would be appreciated …