The speed of light in a non-vacuum medium is slower than the speed of light in a vacuum. The exact speed depends on the specific medium, but it is typically around 75-80% of the speed of light in a vacuum.
The speed of light varies in different mediums because of the interaction between light and the particles in the medium. As light travels through a medium, it is constantly being absorbed and re-emitted by these particles, which causes it to slow down.
According to Einstein's theory of relativity, the speed of light is constant in all reference frames. This means that the perceived passage of time will be different for an observer in a non-vacuum medium compared to an observer in a vacuum. Time will appear to pass slower for the observer in the medium.
No, the speed of light in any medium, including a non-vacuum medium, cannot exceed the speed of light in a vacuum. This is a fundamental principle of physics.
The slower speed of light in a non-vacuum medium can lead to changes in the way light propagates, including refraction and dispersion. These effects are important in fields such as optics and telecommunications.