They say often that light only travels at light speed in a vacuum. That it slows down in air and water. I do not think this is true - correct me if I am wrong as I usualy am :shy: All atoms are vacuum. Light is energy that is not made of atoms, or in other words it is smaller then atoms if you think of it as a particle. This light travels inside of atoms. So it is always traveling in a vacuum and therefor never slows down. I think the measurements are wrong. The light would have many interactions with the atoms that would cause it to take a "bumpy" route to the end of the measuring device. This means that you would read that light travels slower in air or water. If you were able to find an equation for the motion of the light through air or water and find the line integral for the distance traveled you should find that it is still travling at light speed. If all these assumptions are true that means that no one should be able to say that light "slows down" in air or water. It should always be traveling at light speed, regardless if it is in vacuum because it is always in vacuum. This would make things much easier as you always know what speed light is traveling under any circumstances.