Question: why can't you see gases?

Philip Ratcliffe answered on 13 Nov 2014:

Because they’re invisible! Ha ha!
No, seriously. let’s put it the other way around: we only see things that in some way reflect or difract light. That is a light beam hits and object and some part of it gets deflected and ends up in your eye – that’s what “seeing” is.
To do this though basically there must be some free electrons around in the thing we want to see, like in metals and in lots of complex molecules. This is the case of most materials as they are rather complex arrangements of pretty big molecules.
On the other hand, most gases are made up of single atoms, or at best diatomic molecules (like O2 or N2 etc.). In this case the electrons are rather tightly bound into the atom and so don’t interact easily with the photons that make up light.