Whooooosh!

An American physicist has suggested that the speed of light, as traditionally given, may be wrong. And this got me thinking about the speed of light.  We were taught at school that it was 186,240 miles per second (actually a little more).  And then later, when metric measurements became the norm, I had to relearn it as 300.000 kilometres per second (very slightly less actually).  But what doesn't normally get mentioned is that the speed of light varies depending on the medium it is travelling through. The above figures are the speed of light in a vacuum.  In water it's about 25% slower, and in glass some 33% slower. Even in air it is very slightly slower - but by only about 50 miles per second.  So not quite the constant we were always taught.