Confusion about the speed of light

[prane]

So everyone knows the speed of light is a universal constant. However in many phenomena involving light it is useful to think that light is 'slowing down' like light travels slower in glass/ water then it does in a vacuum. How can this be if it is a universal constant?

 

[mathman]

Speed of light in vacuum is universal constant. Light passing through media interacts with media and slows down.

 

[ModusPwnd]

There is a bit about this in the FAQ;

https://www.physicsforums.com/showthread.php?t=511177