# Speed of light is constant in a particular medium

Discussion in 'Physics & Math' started by Saint, Nov 1, 2020.

1. ### SaintValued Senior Member

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Will speed of light vary in different medium?
In vacuum, air, water?
Is that difference significant?

3. ### exchemistValued Senior Member

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The phase velocity of light changes significantly when it passes from a vacuum into air or from air into water. That change in velocity is what leads to refraction.

5. ### Janus58Valued Senior Member

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Significant in what way?
In terms of Relativity, no. Relativity is based on c being invariant, which is the speed light travels in a vacuum.
So for example, in a vacuum, two observer moving relative to each other will both measure the same beam of light as moving at c relative to themselves.
In water however, this would not be the case. Someone as rest with respect to the water would measure the speed of a beam light moving at one speed relative to himself and someone moving at v relative to the water would measure a different value.
But even in this case, c is instrumental.
For the person at rest with respect to the water measures a speed of c/n, where n is the index of refraction. The other observer will measure a speed of (v+c/n)/(1+v/(nc))*

*Velocity addition formula is w = (v+u)/(1+ vu/c^2). In this case, u = c/n, which causes the equation to reduce to the above form.