One glance — do you know which of the bottles is empty?

The other day I was sitting in my conservatory with a friend when I had to take the photo below:

Can you see how one bottle refracts light and the other one does not? What does this tell us about whether there is water in either of those bottles? I met most “normal” people wouldn’t even notice a difference.

I know, I’m a nerd, but I have so much fun “discovering” small stuff like that! :-)