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While researching the cause of refraction, I found that refraction occurs due to the change of speed of light when it goes from one medium to another (according to Huygens principle).

But I cannot get why actually does its speed change when the medium changes?

Qmechanic
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Kartikey
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2 Answers2

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The speed of light in a medium is given by $\frac{1}{\sqrt{\epsilon \mu}}$, where $\epsilon$ and $\mu$ are the relative permittivity and permeability of the medium respectively (these in turn affect how large the electric & magnetic forces in the medium are).

Since the permittivity & permeability of materials vary, it's not surprising that the speed of light varies as well.

Allure
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  • But, if the speed of light changes, doesn't that mean that the light is accelerating? That shouldn't happen, isn't it? – Pratham Hullamballi Feb 01 '20 at 05:13
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    @PrathamHullamballi It doesn’t violate Special Relativity, if that’s what you’re wondering. The speed of light is the constant $c$ only in vacuum. – G. Smith Feb 01 '20 at 05:28
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    @PrathamHullamballi light is certainly capable of accelerating, e.g. when a mass deflects light, the direction vector changes, and there is acceleration. Acceleration doesn't have to involve a change in speed, and even if it does, it does not have to actually lead to a greater speed. – Allure Feb 01 '20 at 05:59
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    @Allure Are you talking about a Newtonian model of light deflection? In GR, particles moving on geodesics through curved spacetime are not accelerating. – G. Smith Feb 01 '20 at 07:00
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    I think I might speak for @Allure who just answered a question posed in term of common meaning, which is not GR. – Alchimista Feb 01 '20 at 09:42
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As Wikipedia’s article “Refraction” explains,

Light slows as it travels through a medium other than vacuum (such as air, glass or water). This is not because of scattering or absorption. Rather it is because, as an electromagnetic oscillation, light itself causes other electrically charged particles such as electrons, to oscillate. The oscillating electrons emit their own electromagnetic waves which interact with the original light. The resulting “combined” wave has wave packets that pass an observer at a slower rate. The light has effectively been slowed down. When light returns to a vacuum and there are no electrons nearby, this slowing effect ends and its speed returns to $c$.

G. Smith
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  • "The oscillating electrons emit their own electromagnetic waves which interact with the original light." can you please tell me if this states that there is photon photon interaction, though that is not possible (only higher order)? – Árpád Szendrei Feb 01 '20 at 04:52
  • @ÁrpádSzendrei No, this is unfortunate wording. The article is using “interact” to mean “superpose”. – G. Smith Feb 01 '20 at 04:59
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    Indirect photon-photon interactions via virtual electron-positron pairs become significant only at very high energy. – G. Smith Feb 01 '20 at 05:01
  • @G.Smith “Electromagnetic waves which interacts with the original light, resulting in a combined wave which has wave packets that pass an observer at a slower rate”?? Doesn’t this leave a whole lot of explaining? How do the two interact? What is your wave? What are your wave packets and how are they slowed down? How about proven science involving the constant speed of light and refraction? As photons travel through the medium they deviate back-and-forth between the atoms do to refraction. The back-and-forth trajectory is longer than a straight trajectory and this longer path takes more time. – Bill Alsept Feb 01 '20 at 06:32
  • @BillAlsept Wikipedia’s microscopic explanation of refractive index has additional information. The math can be found in a variety of physics textbooks. It is “proven science” based on classical electromagnetism and can be understood without ever talking about photons. – G. Smith Feb 01 '20 at 06:45
  • As for your photonic explanation, I have not seen refraction explained this way so I am skeptical that this is a mainstream approach. In any case, whether it is mainstream physics or merely your personal theory, it is unnecessary because refraction is understandable based on classical electromagnetic waves and Maxwell’s equations. – G. Smith Feb 01 '20 at 06:52
  • @G.Smith you either believe in photons or you don’t. It’s far easier to visualize or explain the refraction of a photon. Light waves cannot even be explained physically. What are they if not billions of photons? – Bill Alsept Feb 01 '20 at 06:57
  • Yes, of course I believe in photons. (They are ontologically secondary, however, to the quantum electromagnetic field whose quanta they are.) Can you provide a mainstream reference that quantitatively explains refraction using this zig-zagging-photons approach? How does this model explain the change in wavelength when light enters a medium? – G. Smith Feb 01 '20 at 07:02
  • Light waves cannot even be explained physically. You are entitled to your opinions, but don’t expect physicists to agree with you. – G. Smith Feb 01 '20 at 07:06
  • Why not write an answer to the OP’s question, since you find mine unsatisfactory? – G. Smith Feb 01 '20 at 07:11
  • @G.Smith wavelength is the speed of the photon divided by the frequency. We know the frequency doesn’t change and when the photon exits the medium its still moving at c. Why come up with all those other explanations when it could be so much simpler? – Bill Alsept Feb 01 '20 at 07:21
  • Because the purpose of this site is to provide mainstream explanations as are taught in physics textbooks and encyclopedias. Goodnight. – G. Smith Feb 01 '20 at 07:23
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    @BillAlsept : Your line "As photons travel through the medium they deviate back-and-forth between the atoms [due] to refraction." describes either (1) a diffusion process, which we could detect by photons taking four times as long to pass through twice as much material, which is not observed, or (2) an unprecedented coincidence of scattering only in the very finely tuned directions necessary to get exactly the right amount of "zig-zagging" which we would show is impossible by passing light through an amorphous solid, like glass, which is not observed. – Eric Towers Feb 01 '20 at 16:04
  • @EricTowers (1) your calculation is wrong. Photons do take longer to travel through a medium. As their trajectory deviates their path length increases. In glass that distance increases by a factor of 1.5. The photon is still traveling at c but the overall time as longer. There’s no reason to believe that light speed IS Not constant. Also with a constant speed of light you don’t need to come up with a crazy explanation of how light speeds back up after it leaves the the other side of the medium. – Bill Alsept Feb 01 '20 at 17:32