The common answer to this question is that light refracts because its speed changes in different materials. But this means that the photons has internal attraction between each other. Is that the case? Otherwise is something else like density of different materials the reason why light refracts?

please give a more convincing explanation.

Explanation of refraction

  • $\begingroup$ I don't have an answer but I am interested. I have heard others say the speed of light is constant even in different materials, it just needs to travel farther as it weaves through the molecular structure and making it look like its moving slower. As for the diffraction, could it be caused from the surfaces where the electron arrangements cluster toward interior. This would tend to crowd and interfere with the photon trajectories at the surface. $\endgroup$ Sep 20, 2017 at 16:32
  • $\begingroup$ "But this means that the photons has internal attraction between each other." I kinda see why you might come to that conclusion, but that is not the way to think about what is going on here (which is much clearer in the classical (wave) picture). $\endgroup$ Sep 20, 2017 at 19:03
  • $\begingroup$ @dmckee no it would not mean photons have internal attraction between each other. It would mean they interact with electrons. My point was that the electron arrangements around the surface atoms would be different and more cluster towards the interior. This could cause an effect similar to when incident light is at a larger angle. $\endgroup$ Sep 20, 2017 at 19:11

5 Answers 5


The reason that light travels more slowly in a dielectric is because it interacts with the electrons in that dielectric.

Light has an oscillating electric field, and if any charged particle is in the path of the light that particle will feel an oscillating force due to the oscillating electric field of the light. The result is that the wavefunction of the light becomes mixed up with the wavefunction of the charged particle.

In a dielectric the light interacts mainly with the electrons because (a) they are lighter and more mobile than the nuclei and (b) there are generally a lot more of them. So we are dealing with the interaction of the light with electrons. The result of the interaction is that the light is no longer just light. It is an entangled state of light plus electrons. This entangled state no longer has a zero mass so it travels at less than the speed of light.

The more strongly the light interacts the greater the effective mass and in cases where the interaction is very strong the light can be brought to a complete halt. This can be seen when light interacts with Bose-Einstein condensates, and indeed made the headlines a few years back when experimenters actually managed to bring light to a complete halt.

When the interaction is strong the light plus electrons forms new quasiparticles called polaritons, though for weakly interacting systems like light in glass the quaiparticle description isn't very useful.

  • $\begingroup$ Hi John. I think maybe the question is why does the transition from faster to slower cause the light ray to bend. For that, the explanation I've seen work is that you can treat the wavefront as a collection of point sources, and the superposition of waves self-interferes so that the new wavefront heads in a new direction. So it's not a question of the photons "liking" each other, as he said. $\endgroup$ Sep 20, 2017 at 17:08
  • $\begingroup$ @MikeDunlavey: the mechanism you describe is the classical description. It works because the reradiated light has a phase lag i.e. it is slightly out of phase with the incident light. That phase lag causes interference and the end result s to slow the light. The phase lag is because electrons are massive and take time to respond to the force from the electric field of the light. So at the end of the day it is still basically the mass of the electrons affecting the light. $\endgroup$ Sep 20, 2017 at 17:13
  • $\begingroup$ Do you mean that electrons cause light to bend? $\endgroup$ Sep 21, 2017 at 14:48
  • $\begingroup$ @AhmedElsawy: Hi Ahmed. It's the change in the propagation speed that causes the light to bend. The point of my answer is to explain why there is a change in the propagation speed. $\endgroup$ Sep 21, 2017 at 14:56
  • $\begingroup$ @JohnRennie I appreciate the information but I actually was looking for some explanation for why does the speed causes it to change direction I actually made this animation to try to understand it and it doesn't bend by different speeds: jsfiddle.net/9d3aqum5/1 I hope you take a look at it. $\endgroup$ Sep 21, 2017 at 15:58

This gif shows why with Huygens–Fresnel principle: Huygens–Fresnel principle


  • $\begingroup$ Huygens-Frensel principle is the missing point. +1 $\endgroup$ Oct 11, 2017 at 21:33

This is too long for a comment, and is supplementary to the answer by John.

One should not confuse photons with classical electromagnetic light.

Photons are not light. They are elementary particles in the standard model of particle physics, with spin + or -1 to their direction of motion zero invariant mass and energy=h*nu . Each photon is described by a quantum mechanical wave function. Light emerges from the quantum mechanical superposition of innumerable photons.

But this means that the photons has internal attraction between each other.

No, it is not attraction between photons , but superposition of photon wave functions, which superposed functions give a collective wave function to the emergent light wave. When entering a medium this is modified in the way described in the answer by John, by scatterings with the fields of the electrons in the medium.


The reason for refraction is the different speed of light in different media. This speed is determined by the refractive index $n$, which in turn can be calculated from the relative permittivity and permeability of the material:

$$n = \sqrt{\varepsilon_r \mu_r}$$

These constants simply define the response of a material to an electric or magnetic field. Light (being made up of $E$ and $H$ fields) isn't interacting with itself, but rather with the material it is transmitted through.

So, here it is more useful to look at light in the classical picture as an electromagnetic wave, and not as photons.


There an many ways to look at this problem. I am going to add, light refracts because its path satisfies Fermat's Principle : "light travels between two points along the path that requires the least time, as compared to other nearby paths.”

Given the 2 different speeds of propagation in media 1 and 2:

$ v_2 \sin{\theta_1} = v_1 \sin{\theta_2} $

describes the refraction.


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