Hello, I was recently browsing through on of my physics books, and I came upon the following statement: "Newton was right, light does travel at a constant speed, but it does not travel at the same speed in all media. For example, light travels at a speed in water that is about three fourths the speed of light in a vacuum. The speed of light in air is slightly less than the speed in a vacuum." I found this statement disturbing, because I've always been taught that light always travels at a constant speed. It not that I don't believe that it travels slower in water than in a vacuum, but my question is why does light behave like this? Also, what is the medium through which light travels, if any?