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Suppose you pump humidity (water vapour) in an enclosed tank fitted with lasers in all directions, will the water vapour turn to liquid due to collision of photons and $\text{H}_2\text{O}$ molecules?... My thought is that when the high speed water vapour molecules collide with photons, they will reduce its velocity thus changing them to liquid water... Am I right?

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  • $\begingroup$ What is the color/energy of your photons? Have you computed the allowable change in the momentum of a molecule during a scattering interaction? A search term is “Compton scattering.” $\endgroup$
    – rob
    Nov 29, 2021 at 11:32

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Firstly, water vapour is made up of individual, isolated water molecules.

Over a wide electromagnetic spectrum water does absorb electromagnetic radiation, in particular IR/VIS/UV which you can see here.

But what makes you think that such absorptions can make the water molecules aggregate and condensation to occur?

AFAIK, the excited water molecules shed their absorbed EM energy by an allowed cascade of emissions and nothing else.

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