The speed of light/EM waves in vacuum; as if there was another one in non-vacuum? Q1: is there a speed of a photon other than in "vacuum"? 
Q2: isn't "speed of light in vacuum" misleading?
If I understand, that light moves with speed of light until there is "something in between" (no matter what) (1)
What I ask for, is not a deeply explanation; it's just:  
Children ask me: 


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*"But how can be light slower" if it is a constant?


My explaination is:


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*"Till it collides" (not the deepest answer, I know) 


The question (since I prob. are not at the pulls of new physics): Is there another speed of light  than in "vacuum"  / nothing crossing ?
(1) no discusss, what "what" is
 A: Light is described well by the classical electromagnetic theory and Maxwell's equations.
In this framework, the classical one, the speed of light is constant in vacuum. When light impinges on transparent materials, its speed, classically changes, and this is measured with the index of refraction of the material:

where c is the velocity of light in vacuum and v its velocity in the transparent medium. This can be measured with various methods and thus the speed of light in the material is a measurable quantity.
The classical electrodynamics framework emerges from an underlying quantum mechanics framework. In this framework electromagnetic waves emerge as a confluence of innumerable photons, to which the speed of light in vacuum is assigned.
As the dimensions of quantum mechanical interactions is very small, the photon moves in vacuum until it interacts with a field from the molecules. For transparent materials the probability of the photon scattering elastically is very large in the direction that creates the macroscopically observed index of refraction. In a sense a photon impinging on a crystal is the quantum mechanical boundary problem "photon + crystal" and the solution comes out that the over all speed within the crystal changes due to this interaction.
A: Speed of light is constant.
But in some substances , still transparent , light is absorbed and retransmitted ( with the same properties ) , spending some time. With not well transparent material, things are more complex. Anyway, between 2 obstacles, it's the vacuum and the speed remains constant and maximum.
How many are retransmitted and the specific speed depend on the light frequency ( dispersion , see the prism image ) , the transparency and translucency of the substance and its refractive index.
A: first the speed of light is related to the permittivity and permeability of the medium. changing either one of those values changes the speed of light. 
copper has different values then free space. the speed of light through copper is 2/3rds that of free space. about 1 foot per nanosecond. slightly faster in aluminum, slower in iron. this can be measured with an oscilloscope.
freespace is not perfectly even. there are variants.
these are simple facts that can be imperially verified and are the basis of electromagnetic communication. it is a provable fact that free space has an inductive reactance of 376 ohms. and that should start you thinking ... how can something that is devoid of all things have a measurable property. 
