# Thread: What is the mechanism of refraction?

1. I was always taught that when light enters a second medium that has a different index of refraction than the first, the direction of the light will change assuming it enters at an angle that is not normal to the interface (Snell's Law). However, I have not been able to find the answer to my question anywhere, even with some of my professors. What mechanism actually causes light to change direction? Some people I talked to said that it is because of the change in speed, but that is not a mechanism. To that I respond, "When I slow down in my car, I do not change direction," and that is pretty much the end of that conversation because no one I know has an answer. I know that my analogy is probably not good, but it shows to them that a change in speed does not have to mean a change in direction. Any help?

2.

3. Originally Posted by dcOSU
I was always taught that when light enters a second medium that has a different index of refraction than the first, the direction of the light will change assuming it enters at an angle that is not normal to the interface (Snell's Law). However, I have not been able to find the answer to my question anywhere, even with some of my professors. What mechanism actually causes light to change direction? Some people I talked to said that it is because of the change in speed, but that is not a mechanism. To that I respond, "When I slow down in my car, I do not change direction," and that is pretty much the end of that conversation because no one I know has an answer. I know that my analogy is probably not good, but it shows to them that a change in speed does not have to mean a change in direction. Any help?
Here's an analogy that might help. Consider battalion of soldiers marching in rank and file. The first line of soldiers reaches a muddy field which slows how fast that they march. If they hit the field straight on , they all slow down in unison and march on.

But what if they hit at an angle? One end of the line hits the field first and slows down while the rest of the line marches on at normal speed. However this will stagger the line and the soldiers must march abreast and the soldiers that haven't reached the field can't slow down. The only way for the to remain abreast is for the line to turn to accommodate the the slowed down soldiers.

As far as your cars goes. If you hit a driving surface that slows you down at an angle, you will turn. If the Left wheel hits first, slowing the left side of the car the right side will tend to continue on and the car will pull to the left. You would have to actively correct to prevent it.

4. Thank you for the reply. I have heard of the line of soldiers before, but never applied it to my car analogy. But the analogy only works for something with a horizontal width, doesn't it? Can I not think of it as a beam of photons entering, one after the other? In that case there would be no horizontal line that would turn, and it seems that they should just go straight through. And I believe we are capable of producing single photons, so what would happen in that case?

Also, I know that the speed changes because the fluctuating electric field causes the electron shells of the atoms in the medium to oscillate, which in turn interacts with the light and slows it down. Can this somehow explain the bending?

5. A photon certainly has some dimensions, though. So, one side of it would still make contact before the other. One soldier sticking one foot in mud and the other left out of it would have a hard time walking straight. Using equal amounts of energy to lift his foot and move it, one foot would make a bigger stride and he would be forced to list toward the slower medium.

6. Originally Posted by dcOSU
I was always taught that when light enters a second medium that has a different index of refraction than the first, the direction of the light will change assuming it enters at an angle that is not normal to the interface (Snell's Law). However, I have not been able to find the answer to my question anywhere, even with some of my professors. What mechanism actually causes light to change direction? Some people I talked to said that it is because of the change in speed, but that is not a mechanism. To that I respond, "When I slow down in my car, I do not change direction," and that is pretty much the end of that conversation because no one I know has an answer. I know that my analogy is probably not good, but it shows to them that a change in speed does not have to mean a change in direction. Any help?
To add to the line of soldiers analogy, think about how a tracked vehicle like a tank or a Caterpiller tractor is steered -- by braking the track on one side. That is ditectly changing direction through speed control.

7. Stop thinking of light photons as just a particle. In this case the wave interpretation works much better. The light, even a single beam, will have a wavefront which will interact across a certain width. The wave of 'marching soldiers' will then bend towards the normal.

8. Now..... I'm pretty sure this is right, but electromagnetism is kind of a weak area for me right now. This is how I understand the problem now, anyway.

Think if you threw 4 evenly spaced marbles toward a junction between two mediums, where the second offers more resistance than the first, in such a manner so the 4 marbles are traveling parallel to each other, and they approach the junction from an angle.

So:

. -------> /
. ------> /
. -----> /
. ----> /

After they hit, the marbles aren't parallel to each other anymore.

-----/--------->.
----/--------->.
---/--------->.
--/--------->.

I think the reason is that, unlike the marbles, the photon isn't moving because of inertia. It's a propagating wave, and it has to move where the wave propagates toward, which is always the direction perpendicular to both its magnetic and electric fields. If we assume these photons were polarized and in phase with each other prior to hitting the junction, and that the magnetic field component were pointing straight up and down at that time, then now the point where their magnetic fields reinforce the most will be along a line drawn through the dots. And, if the magnetic field faces in a new direction, then the wave must now propagate in a new direction.

The part I'm sure about is that a photon has to travel in such a direction so that the marbles always remain parallel. Just my explanation for the reason why might be off.... I might have to come back to this thread and a few weeks and explain how I was way off...

9. The mechanism of transmission of light is also very important to be understood in physics. Like if photoelectric effect wouldn't ave been explained then, we would have seen no solar cells. similarly the way world is marching towards optical computers, its very much necessary that we understand the basics, so that we don't complicate when it was so easy to understand. One way you can understand how a photon travels (see the last image of the link The Dual Nature of Light as Reflected in the Nobel Archives) is that it does not travel at a lower speed in denser medium but travels a larger distance with its original speed, thus taking a larger time to travel. Richard Feynman explained the same but he said that the photon is absorbed and remitted, if this is the case then after absorption the photon has all the possibilities to get scattered in all direction. If one explains all the phenomenons by photon model (as it satisfies all the properties of light in photoelectric effect) then his work will change the way we look at light. As Einstein said, imagination is the key, that's where even supercomputers fail. I wish that you can now imagine the refraction process after understanding the concept of larger distance traveled by light.

10. Originally Posted by dcOSU
Also, I know that the speed changes because the fluctuating electric field causes the electron shells of the atoms in the medium to oscillate, which in turn interacts with the light and slows it down. Can this somehow explain the bending?
As you can tell from the variety of responses you received, identifying "the" mechanism is somewhat a function of taste. If we go down a level from the "marching soldiers" analogy, you are right in recognizing the role of charges. Both reflection and refraction are due to the re-radiation of light by a material/medium. And for that, you need to have mobile charges. The free electrons in a metal explain why metals make good mirrors, and the movable dipoles in dielectrics explain why they refract well. When you accelerate charge, they emit light (most of the time; there are a few exceptions, but we'll ignore those here). An incident EM wave exerts forces on charges; the latter move in consequence and that motion produces EM waves (light). The induced wave superposes with the incident wave to produce a net wave with an amplitude and phase (delay) that differ from those of the incident wave. Different materials produce different responses to incident waves, thus giving you differing amounts of amplitude change and delay. In the case of refraction, you are most focused on the delay produced. That delay gives you the marching soldiers, bending, Snell's law, etc.

11. Don't use the soldier analogy, it leads to misconception (like they are related to one another). No light is a a perfectly parallel group of photon.
Feynman respond to your question in very simple ways, here is quick Google picture search (Feynman clockwork)
Refraction.jpg

It is a quantum view. so everything is a point, the example below is for a hypothetical surface of no width. It can be expanded quite easily to any width of any matter. All that changes are the infinite little clocks timings associated with every hypothetical path that a photon can take. Those add into a sum vector which is your refraction or reflection number. So the "mechanism" is that each photon hitting(interacting) a light- non-absorbent atom (well electron) will be delayed.
Ho, and yes, you must also swallow that every single photon has to pass through every paths AT ONCE .

That's the beauty of it. A well deserved Nobel Prize IMO

12. Refraction is Interesting. It has not been understood. Maybe you guys can solve it.

What mechanism actually causes light to change direction?
Excellent Question. dcOSU. You are a smart lad.

Think. Think hard. you are on the right track.

13. Excellent Question. Now keep thinking. Think hard. You'll get it.

I don't have the answer for this, but I feel this one is yours

14. This is related to constructive & destructive interference. Light will appear at point(s) of constructive-interference but disappear/absent at point(s) of destructive-interference. -ie: wave on the pond.

What exactly happen:
light pass thru the medium (glass or water) and as it pass: it create a successive spherical wavefront which constructively build-up at certain direction but is destructive elsewhere (so light appear at this point(s) and appear to move in a different direction than the angle which it enters the medium). This is a 3-dimensional interpretation of Huygen's principle which was first described for 2D wave (ie: wave on pond).

Also, incase you are wondering:
the reason why bluer light is diffracted more than redder light (ie: why blue it at bottom of the rainbow from a prism) is because bluer light has higher frequency hence it interact more with the medium (glass or water). Thus it create more spherical wave when it enter the medium and produce a completely different set of constructive interference point(s) (this cause different diffraction angle). -for more info search for Huygen's principle (Source: university physic textbook)