The YouTube video How to control light with water shows that light can be 'directed' by water. Recently, there has been things about universities making 'hollow-fibre'. What happens if you filled the tubes with water? Would this be more cost-effective? Is there any cons to it? I don't know much about the subject, but this confuses me.
Light is slowed down when it travels in a transparent medium. (Only the speed of light in vacuum is constant). If Water were the medium, as it is denser than air, the light would travel slower than it does in the air. This is the point of using hollow fibres, the air is less dense than the glass fibre previously used.