Greetings, When I think of light being "transmitted" through air, I have always used the idea that the light is not interacting... it is just passing through. However, I had a student ask me, if transmitted light is not interacting with air molecules, then why is the speed of light slower in air, relative to a vacuum. Being a chemist, I had to think about this... Clearly, light must be interacting with air, else the velocity would be the same as in vacuo. So, two questions: 1) What is the nature of the interactions of visible light with say, O2 or N2, as it is being propagated along and 2) Given this, is it even correct to say that light is being "transmitted" if there are these interactions? Thanks!