Question: why are most gases transparent?

Mike Lawton answered on 15 Jun 2015:

Hi Mike here. The answer lies with the density of molecules, i.e. how closely they sit relative with each other. With a gas, the number of molecules is small – so small in fact that they don’t (normally) interfere with the visible light entering your eye. If we increase the pressure, then the number of molecules in a given volume increases to the point you can see them.

Picture a transparent cigarette lighter – you can see the butane gas as a transparent liquid because the molecules are so squished together. Reduce the pressure by pressing the lighter and the butane becomes an invisible gas as the molecules push away from their neighbours.

BTW – really dense molecular arrangements gives us solids. The more dense their packed, the heavier the material. At one extreme we have a neat material called Aerogel that is 90% air and feels impossible light in your hand. At the other end, we have a metal called Osmium that’s twice as heavy as lead. A carrier bag full of it would weigh more than a family car!