What’s the refraction of air?

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Looking into water, objects are visually displaced by light refraction.

Similarly, looking out of water from within also produces refraction.

Is this entirely because of the refraction by water, or does atmosphere produce its own refraction? Are we evolved to perceive our atmosphere as “clear,” but if an alien from a planet with a different atmosphere came to earth, would they be thrown by our atmosphere’s unique refraction? Or are the gases that make up our atmosphere non-refractory?

If we could somehow survive (and see) in the vacuum of space, would all objects appear visually displaced, like they would looking into water?

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The refraction happens when you change from a medium of one refractive index to another. So it is not the air or the water creating the refraction but both, or rather the difference between them. Vaccuum have a refractive index of 1, air is 1.000293 while water have a refractive index of 1.333. So it is this difference in refractive index which produce the refraction you observe. The refractive index of air does depend on the temperature and humidity so you can see refraction between different layers of air with different temperatures. This is what causes the wavy look of hot air. As for humans we are adapted to the refractive index of air in that our eyes depend on the refraction as light goes from air into our eyes to focus the light onto our retinas. This does not work well in water which have a different refractive index causing us to see blury. Animals which have adapted to seeing under water have similar issues as they come up on land.

Looking into water is a large change of index of refraction, which causes a large change in the angle of the light when going into it.

Air is very close to the refractive index of vacuum, only slightly higher. Small changes in index lead to small changes in angle. But even that small change can matter: temperature will change air’s refractive index, and very hot air just above the surface of the road will steer light that passes through it. At a very shallow angle, it steers it enough to bring it up toward your eyes and now you have a mirage. The light from the sky is bent upwards by the refractive index gradient of the hot air.

Your eyes’ optical system is tuned to work in air, which is why things are blurry when looking underwater. But as soon as you put on goggles that leave some air right in front of your eye, everything looks fine. Most gases are going to be very close to the refractive index of our mix that we call air, so there won’t be much difference seeing in a different gaseous atmosphere or vacuum.