Question: Why are most gases invisible?

Camilla Weiss answered on 15 Jun 2015:

Hi Beth,

Good question. Gases aren’t actually invisible and there are some gases, like you said, which we can see. Iodine gas for example is purple and Chlorine gas is greenish. The main reason we don’t see gases is actually because of how our eyes absorb light, or electromagnetic radiation. We can only see a very small range of frequencies of light – the visible range (think of a rainbow as the range of light frequencies we can see). In order for us to see something with our eyes an object needs to reflect light in this range. Gas molecules, which are too small to see, don’t react with light in the visible range which is why we can’t see them (this is to do with how electrons interact with photons if you wanted to go into more detail). If that doesn’t make sense, make me explain more clearly!