# Make air reflect light

Is there any way in which air can be made to reflect light? By making air denser or by any other way? The task is to project images in air...

• Did you mean heliodisplay Jun 3, 2014 at 15:33
• @RijulGupta, well yes, similar to that. Heliodisplay condenses air so that images can be projected on that condensed air. But its major disadvantage is that it needs controlled environment with no air disturbances. Jun 3, 2014 at 16:29

If the wavelength of light is much smaller than the dimensions of obstacles; we don't encounter diffraction.$_1$ The wavelength of Visible light is $4\times10^{-7}$ to $8\times10^{-7}$ meters, which is greater than the radii of the most atoms, i.e around few $10^{-9}$meters.$_2$ So, you can expect diffraction and not reflection.

Some of the molecules might have greater dimensions than the wavelength of light, but all the molecules won't reflect light, instead they might absorb. Even if some of the molecules do reflect, they appear so small, which will be difficult to detect from your naked eyes. Over all, the air in front of you appears transparent.

Even if you make the air denser, it is important to have most of the molecules which reflect the visible light rather than absorbing. I would say it is possible but not impossible. Some times smoke is spread to make a laser beam visible in college laser experiments.

Credits: $_1$Karnataka 2nd PUC Physics textbook-2013 Edition-Page No.367 $_2$Modern's ABC of Physics Class XII-2013 Edition- Page No.789. Some modification has been done.

• "Some times smoke is spread to make a laser beam visible in college laser experiments" That is tyndall effect and not reflection! Jun 3, 2014 at 17:45
• Diffuse reflections are nothing but reflections undergoing scattering. And Tyndall effect involves scattering. Jun 3, 2014 at 18:05
• I know that, it was just for clarification(for other readers) that it is not what someone would outright call reflection. Jun 3, 2014 at 18:09
• @RijulGupta: Well done! Jun 3, 2014 at 18:15
• Was that sarcasm o.0 ? Jun 3, 2014 at 18:16

Air does reflect light. That's how mirages come into being. Granted, this reflection takes place at relatively low angles of incidence and at a relatively sharp boundary between air layers of different temperature (and thus density and thus index of refraction), but it certainly happens.

• "Mirages" are not reflection! JohnRennie also posted this answer which he himselfdeleted because it was wrong! Jun 3, 2014 at 17:29
• @RijulGupta I'm not sure why JohnRennie deleted an answer: while the effect is essentially "total internal reflection," that's still a boundary reflection effect. Not all reflection involves bouncing off a high-n (or n = -1 mirror) layer. Jun 3, 2014 at 18:03
• "Mirages" are a result of refraction + total internal reflection, and I did not say "wrong" as in concept, just in context. The OP agrees(see in comments) that he is talking about the holographic/aerial 3d displays. Since that is the question, "mirages" are not the correct answer. Jun 3, 2014 at 18:14
• @CarlWitthoft, its true that reflection happens at a very low degree in air, and the task is to increase the refractive index of specific region in air, so as to project images on it. Also, instead of increasing refractive index of air, some other approaches can be like using Plasma excitation... Jun 4, 2014 at 4:16

See this

This is taken from here

This is an article from 2011, but I believe the idea is not widespread.

It may not be called reflection on air, but it is as close as you can get! What is done is that high powered laser are pulsated to generate light points by making plasma excitation in atoms of oxygen and nitrogen in the air.

The name for this phenomenon is static volume volumetric display(See last paragraph in the wiki page)

• Yes, by far this Aerial 3D system is the most promising among others. But it does so by Plasma excitation, which has not been explored a lot yet. There are many ways in which plasma excitation can be done. Don't know which one is used in Aerial 3D system. Do you know which? Because that again can lead to some other issues... Jun 4, 2014 at 3:59