Why is air invisible?

Swetasuria

Is it because the atoms constituting air are far apart?
Or is it because they don't give out radiations in the visible range?

Vladimirr

Technically it's not invisible, it's just very close to transparent. If you look up at the sky in the daytime, you don't see stars, you see blue. This is due to Rayleigh scattering.

But as to the question of "why can't I see air molecules floating around the room" - it's because the particles are smaller than the wavelength of visible light.

Khashishi

Air is very sparse compared with liquids and solids, so there's just much less material to block light. Secondly, air is very uniform and homogenous so there are no edges to cause reflections and scattering, except in the extreme case of a explosion shockwave. For the same reason, still water is mostly invisible except at the surface, while frothy water is very visible. Thirdly, the molecules that comprise air are very simple and don't have a lot of lines in the visible range. This is in contrast to complex organic molecules with a lot of vibrational and rotational modes. Also, our eyes evolved to see in a range where air is transparent.