Question: Why Does Compressed Air Cause Refraction?

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    Refraction
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Discussion Overview

The discussion revolves around the phenomenon of refraction caused by compressed air, particularly in the context of an aerosol can. Participants explore the relationship between pressure, density, and refractive index, drawing parallels to heat haze effects observed over hot surfaces.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant notes that the visible refraction from the jet stream of compressed air is similar to heat haze, suggesting that the refractive index of a gas varies with pressure.
  • Another participant explains that the refractive index of the compressed air differs from the surrounding air due to changes in density as the gas expands.
  • A question is raised about the possibility of creating a lens with a specific refractive index by using a shaped container filled with pressurized gas or liquid, considering the effects of volume and pressure on refractive index.
  • A response indicates that while water does not compress significantly, changes in refractive index can occur with temperature or dissolved substances, and that optical methods can measure small differences in refractive index for various applications.

Areas of Agreement / Disagreement

Participants generally agree on the relationship between pressure, density, and refractive index, but the discussion includes exploratory questions about the practical implications and applications of these concepts, indicating that multiple views and uncertainties remain.

Contextual Notes

There are limitations regarding the assumptions about the behavior of gases and liquids under pressure, as well as the specific conditions under which refractive index changes occur. The discussion does not resolve these complexities.

RagingPineapple
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I have a question!

On my desk is a aerosol of compressed air, for blowing dust away and stuff.

If you look carefully, the jet stream of air causes a visible refraction against the background. Why is this?

I'm assuming it's based on the same way that a heat-haze causes refractions over roads and cars.
 
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Yes it's similair - the refractive index of a gas depends on it's pressure.
On the road the hot air has lower pressure than the surroundings, making a lens.
In the compressed air can the expanding compressed gas has a differnet density to the room air.
 
So taking into account the refractive index of any transparent container it would require, would it be possible to create a lens with a specific refractive index simply by creating a shaped container of pressurised gas/liquid?

Just curiosity really :)

Like with water, if it were possible to forcibly inject water into an already full container, thereby compressing what was already there, would you visibly see the refractive index of the water start to alter as the volume in the tank increased?
 
Yes - it would be fairly slow to respond though.
Water doesn't compress much so there isn't a big change in refractive index with pressure - it does change with temperature or other stuff disolved in it. If you add vodka/gin to water you can see the difference in refractive index as they mix.

The techniqueis uesed the other way around to measure density of a material.
It's easy optically to measure extremely small differences in refractive index and this is used to determine the concentration of things like salt/sugar in a liquid or to match a sample of glass by putting it in a tank of oil and heating the oil until it matches the refractive index of the glass.
 

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