Re: I suppose at least the RN have one ship that works
It's Friday, so just a gentle little point - light travels at only one speed. It doesn't vary according to the medium.
Think of it this way: a beam of light travels from point A towards point B. It's going at the speed of light - it can do no other. Later, a beam of light, also traveling at the speed of light, arrives at point B.
When the time is measured, it seems to have taken too long, and therefore was 'slowed'. It wasn't - it can't be. It took the shortest possible route, as light always does. But, roughly speaking, the route was longer than imagined. Off for a beer now.