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?

  • $\begingroup$ 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. $\endgroup$ Dec 6, 2017 at 19:32
  • $\begingroup$ 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. $\endgroup$ Dec 6, 2017 at 19:47
  • $\begingroup$ but i read " IT IS IMPOSSIBLE FOR A LIGHT PARTICLE NOT TO GO AT SPEED OF LIGHT " $\endgroup$ Dec 6, 2017 at 19:53
  • $\begingroup$ 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". $\endgroup$
    – NickD
    Dec 6, 2017 at 19:58
  • $\begingroup$ “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. $\endgroup$ Dec 6, 2017 at 20:25

1 Answer 1


Einstein's two postulates of Special Relativity: 1. The laws of physics are invariant in all inertial systems. 2. The speed of light in a vacuum is the same for all observers, regardless of the motion of the light source relative to the observers.

So the context is important that the speed of light in a vacuum never changes.

From the wave equation (http://maxwells-equations.com/equations/wave.php), c = (mu*epsilion)^(-1/2), where mu (epsilion) is the permeability (permittivity).

The speed of light does slow down in a medium, as described by the above equation. As light passes through a medium, the electric and magnetic fields of the photon oscillate electrons in the medium (at the frequency of the light wave), and those electrons respond by radiating electromagnetic waves at the same frequency, but with a time delay, and these EM waves constructively and destructively interfere with the incident light wave, reducing the light wavelength but maintaining the light frequency, and effectively reducing the phase velocity of light(https://en.wikipedia.org/wiki/Refractive_index#Microscopic_explanation).


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