# Problem with speed of light in different material [duplicate]

I read somewhere in a relativity article that light speed is constant and it is impossible for a photon to not travel at speed of light. Now i study in year 10 and today i studied that light speed can change in different material like glass and water. so how does that work?

• The constant speed of light that we are all familiar with: $c=299792458\text{m s}^{-1}$ only applies to light in a vacuum. The speed of light in other materials is lower. Dec 6, 2017 at 19:32
• The speed of light (c) isn't actually the speed of light, it is the speed of causality. The actual "speed" of light can be anything depending on the material light is traveling through. Light travels at c in a vacuum which where people got the name from. Dec 6, 2017 at 19:47
• but i read " IT IS IMPOSSIBLE FOR A LIGHT PARTICLE NOT TO GO AT SPEED OF LIGHT " Dec 6, 2017 at 19:53
• It should say "it is impossible for a light particle in vacuum not to go at the speed of light in vacuum". When discussing relativity, the "in vacuum" parts are usually elided, because they are supposed to be "understood from the context". Dec 6, 2017 at 19:58
• “It is impossible for a light particle not to go at the speed of light” does not mean that the speed at which light travels cannot change, it just means that it must always equal the ‘speed of light’. This speed is not fixed to c for all substances—only for light in a vacuum. If the medium changes, the speed of light also changes, and light must now ‘always’ travel at the (new) speed of light. Dec 6, 2017 at 20:25