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Why is the speed of light different when traveling in different materials?

  1. Dec 6, 2012 #1
    In my physics class, we learned that light moves slower when traveling in different materials like water or air. However, it is the fastest in a vacuum. I was wondering why this happens. I read something about a refraction index in the different materials. For example, water has a refraction index of 1.3 or something. So, the speed of light in water is technically c/1.333....;the problem is, I still don't understand WHY this happens. Any help would be really nice. Thx!
  2. jcsd
  3. Dec 6, 2012 #2
    Isn't the speed of light a constant. Having said that if the light takes longer to pass through say glass then it must be travelling further.
  4. Dec 6, 2012 #3
    Check out the FAQ;
    https://www.physicsforums.com/showthread.php?t=511177 [Broken]
    Last edited by a moderator: May 6, 2017
  5. Dec 6, 2012 #4
    The speed of light is always C. Due to the absorption and reemission of the photon as it passes through the atoms of the material, there is a slight delay in transfer time through the material.

    Edit: It is always C in a vacuum...
    Last edited: Dec 6, 2012
  6. Dec 6, 2012 #5
    Only the speed of light in vacuum is constant.

    If a wave propagates in a medium it drives the electrons to oscillate around the core. Due to this movement of charges new waves are excited at each atom site.
    All the waves taken by themselves oscillate with the vacuum speed of light, however when the scattered waves interfere with the incoming wave they produce a new field pattern which also is a plane wave, but with a smaller "apparent" phase velocity.

    I just saw the reply about the FAQ entry. The FAQ answer actually is quite confusing to me, because it attributes the refractive index to a lattice response and explains something that sounds to me like Raman scattering. While this exists, it is more of a special phenomenon. Mostly the refractive index is treated as an electronic response (some keywords: drude model, lorentz model, atomic dipoles, clausius-mossotti etc). For example, the explanation that i posted can be found (together with the math) in the Feynman lectures.
    Maybe i misinterpreted the FAQ answer, or maybe there is also some lattice contribution to the refractive index (not raman scattering), which i did not know about.
    It would be nice if this could be clarified.
    Last edited: Dec 6, 2012
  7. Dec 6, 2012 #6


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    Classically, light is an electromagnetic wave. When the oscillating electric field goes through a dielectric material, the electric field causes the charges in the material to move, which creates a temporary dipole moment which opposes the electric field. The electric field is generated by a changing magnetic field, but because of the material, the electric field is weaker. Because of conservation of energy, the energy of polarization of the material contains the rest of the energy, and this energy is released back into the wave. But it slows down the propagation of the wave, as the light wave has to push the charges in the material around as it passes.

    For dispersive media, "speed of light" is ambiguous. Do you mean the phase velocity, the group velocity, or the signal velocity of light? The phase and group velocities can exceed c, but the signal velocity is always <= c.

    Quantum mechanically, the photon becomes a quantum of oscillation in the electromagnetic field superposed with oscillation in the charges in the material. A "particle" in quantum mechanics is allowed to be built out of different pieces, like Frankenstein's monster. I don't know if this means that the photon picks up a rest mass. It's beyond my ken.
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