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When comparing the speed of light through a material medium as opposed to through a vacuum, the speed through the material (e.g. glass or air) will be slower.

Is this because:

  • The light has to bend around the atoms but still travels at the speed of light. Because the distance is longer it can't arrive in the same time period.
  • The atoms change the space around them so that light travelling in a straight line slows down from our frame of reference. From the frame of reference around the atom, light is travelling at the "regular" speed of light. The distance is the same, but the light has experienced time dilation.

Which of these (if either) is true?

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Here is a classical picture that gives a qualitative feel for why light would be slower. That is not to say that the mechanism here is right. This is actually more an explanation of why sound travels slower than light. But assuming light interacts with atoms and this takes time before moving on to the next atom, you can see enough similarity.

Light travels at the speed of light. So do the electric and magnetic forces between atoms in a solid.

Suppose you have some sort of disturbance that travels from atom to atom in a crystal. The disturbance might push an atom out of place (sound), or distort its electron cloud (light) or something else.

The news that an atom or electron is displaced travels at the speed of light, so neighboring atoms begin to feel a change in forces that quickly.

But neighboring atoms/electrons have mass. They begin to accelerate when they begin to feel the changed force. It takes a while before they have been displaced as far as the original atom/electron. So the disturbance propagates slower than light.


We can do a really crude approximation to check on this. We suppose that sound propagating through Copper displaces atoms, while an electrical pulse displaces individual conduction electrons. We suppose that the electrical forces holding atoms in place are about as strong as the forces holding electrons in place.

An proton or neutron is $1800$ times more massive than an electron. A copper atom has about $63$ protons and neutrons. It is therefore about $110000$ times heavier than an electron. We might expect the acceleration of a conduction electron from an electrical pulse to be about $110000$ times greater than the acceleration of an atom due to a sound pulse. We might expect the speeds of propagation to have a similar ratio.

The speed of sound in a metal like copper is ~5000 m/s. (Depending on factors such as if the copper is rolled or annealed. See The Engineering Toolbox.) The speed of an electrical pulse in a copper cable is $40$% to $75$% the speed of light in a vacuum. See Wikipedia. So an electrical impulse is actually $24000$ to $36000$ time faster. This is in the ballpark of what we might expect.

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It is important to understand that light obeys the classical electrodynamics of Maxwell's equations , is usually modeled by rays. Whereas transparent solids are made out of the quantum mechanical entities we call atoms , quantum mechanically bound states of electrons and nucleons. The solids form lattices (drawing of single block) which again obey quantum mechanical equations.

In the classical formulation, light going through a transparent lattice follows the rules described in the other answer, the classical electrodynbamics, and there the velocity can be very low, and can be modeled with ray optics.

BUT we know that light is composed out of photons, which as zero mass particles always travel at velocity c in vacuum. Photons as quantum mechanical entities have quantum mechanical solutions for going through the lattice. I have an answer here that is relevant.

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