Separating colours and slowing/accelerating waves

[keokeo22]

I don't know if this is the right forum to post this, but I was just wondering if a light wave could be slowed down or speeded up in frequency somehow to change dinamically its color (maybe just a red wave separated through a lens filter) in open air.

Is this possible and how one could achieve this?

 

[sophiecentaur]

The problem with this is that the Frequency of an electromagnetic wave that stays the.same when it crosses any boundary or goes through any medium. The Photons have a certain energy and that's it.

The only way to change that frequency is by moving the source or the observer very fast towards (blue shift) or away (red shift). This is the Doppler shift. To get an appreciable 'colour change' you would need to have relative motions quite near to the speed of light.
Whilst we can get an audible change in the tone of a siren when a police car approaches us and then recedes, light travels about a million times faster so the whole thing would need to scale up.

However, with electronic systems, it is possible to measure quite slow relative speeds using Doppler shift of radio (microwave) signals.

What you can do, with light, is to start with UV and shine it onto certain materials, called Phosphors. These absorb the UV and can emit visible light. They are referred to as frequency changers but they don't really do what you were asking for,

 

[keokeo22]

Thank you, I was just wondering why air-screens wouldn't still come up in the tech industry. The maximum I've seen has been achieved by japanese guys (http://www.physorg.com/news11251.html) but it's not a 3D monitor yet, no colors and pretty dangerous for skin too