# How can light go back to it's original angle after being refracted?

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We just learned in physics today that the reason light changes direction when it passes through different mediums is because it's speed changes. When it leaves the new medium and travels back into the original medium, it goes back to its normal direction and speed. How can it go back to it's original speed if it has already slowed down? Unless there is something constantly propelling light so it can regain the energy it lost. My teacher couldn't answer it, she also said because a when a light's angle changes, all that is changing is it's speed.

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How can it go back to it's original speed if it has already slowed down?

It slows down due to interactions with the electrons in the medium. The photons themselves always move at c, but they are absorbed and reemitted by the electrons in the material.

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We just learned in physics today that the reason light changes direction when it passes through different mediums is because it's speed changes. When it leaves the new medium and travels back into the original medium, it goes back to its normal direction and speed. How can it go back to it's original speed if it has already slowed down? Unless there is something constantly propelling light so it can regain the energy it lost.

Ah, but that's just it — it didn't lose any energy.

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I found another version of my question on another forum and one of the answerers said that light is always traveling at c, just when it hits a medium, it bounces around off of the particles in the medium, so it appears to have slowed down. Are the interactions with electrons you're talking about the same as light bouncing around the particles in the medium?

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Get a new teacher.

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You may imagine photons absorbed and re-emitted if you wish... It's more than an image, it's also a means for numerical predictions - a model, in other words. BUT beware one book has spread a false idea. Re-emission takes no delay. It's the electric polarization of the medium (usually the molecules) reacting to the electric field that makes photons slower.

As well, absorption and re-emission don't mean that light is scattered. It can stay perfectly coherent and focussed, for instance in a telescope. Hence stay away from the image of bouncing light. For instance, a gas more compressed slows down light more, though bounces would change the direction by just as much.

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About energy: you seem to imagine the photon's energy like a kinetic one, and though it's not false, this image can be misleading. The photon's energy is uniquely linked with its frequency, which doesn't change from one medium to an other.

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You may imagine photons absorbed and re-emitted if you wish... It's more than an image, it's also a means for numerical predictions - a model, in other words. BUT beware one book has spread a false idea. Re-emission takes no delay. It's the electric polarization of the medium (usually the molecules) reacting to the electric field that makes photons slower.

No. Photons travel at c. The polarization of the medium affecting the electric field is the classical explanation and that indeed causes a slowing of a wave, as it must, because both the classical and quantum explanation predict that the light will slow down.

For instance, a gas more compressed slows down light more, though bounces would change the direction by just as much.

A compressed gas will have a higher index of refraction and cause a greater bending of light.

I found another version of my question on another forum and one of the answerers said that light is always traveling at c, just when it hits a medium, it bounces around off of the particles in the medium, so it appears to have slowed down. Are the interactions with electrons you're talking about the same as light bouncing around the particles in the medium?

Light slows down — the speed in a medium is c/n. Photons, however, always travel at c.

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It slows down due to interactions with the electrons in the medium. The photons themselves always move at c, but they are absorbed and reemitted by the electrons in the material.

Above explanation can be true only if we experience that ionized hydrogen, alone protons without electrons, is not slowing down photons..

Is there some experiment showing this?

Otherwise if proton can absorb photon then you should say:

"It slows down due to interactions with the electrons and protons in the medium. The photons themselves always move at c, but they are absorbed and reemitted by the electrons and protons in the material."

ps. Proton can even absorb electron and some additional energy and convert to neutron..

http://en.wikipedia.org/wiki/Electron_capture

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Above explanation can be true only if we experience that ionized hydrogen, alone protons without electrons, is not slowing down photons..

Is there some experiment showing this?

Otherwise if proton can absorb photon then you should say:

"It slows down due to interactions with the electrons and protons in the medium. The photons themselves always move at c, but they are absorbed and reemitted by the electrons and protons in the material."

Technically it's absorbed by whatever material into virtual excitation states. You're splitting hairs here that are beyond the level of the question that was asked.

ps. Proton can even absorb electron and some additional energy and convert to neutron..

http://en.wikipedia.org/wiki/Electron_capture

And this has nothing to do with the OP.

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As I understand on the macroscopic scale, light would bounce off at the same angle, but in the atomic scale, doesn't it stand that you cannot predict exactly which angle an electron will emit a photon at? So how exactly is the angle the same considering that?

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As I understand on the macroscopic scale, light would bounce off at the same angle, but in the atomic scale, doesn't it stand that you cannot predict exactly which angle an electron will emit a photon at? So how exactly is the angle the same considering that?

Virtual-state excitations inside the medium absorb neither energy nor momentum, so the photon has to continue along the same path.

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Virtual-state excitations inside the medium absorb neither energy nor momentum, so the photon has to continue along the same path.

"Along the same path" seems to imply that the photon would pass right through the electron without alteration, but light is obviously altered in non-vacuum states which is why it seems to take longer to reach a certain point in something like air even if it does not excite those particles.

Edited by SamBridge
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Technically it's absorbed by whatever material into virtual excitation states. You're splitting hairs here that are beyond the level of the question that was asked.

I just pointed out obvious thing that it's not just electron that's absorbing photons... Somebody from that statement can conclude that only electrons are absorbing photons, and protons/atom cores don't.

Actually I should also mention positrons, and antiprotons as well as electrons and protons, to complete list.

Edited by Przemyslaw.Gruchala
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"Along the same path" seems to imply that the photon would pass right through the electron without alteration, but light is obviously altered in non-vacuum states which is why it seems to take longer to reach a certain point in something like air even if it does not excite those particles.

The alteration is the arrival delay, and also the shift in direction that takes place at the interface, which is the result of conservation of energy and momentum. But that's also a complicated issue (search for the Abraham-Minkowski paradox)

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The alteration is the arrival delay, and also the shift in direction that takes place at the interface, which is the result of conservation of energy and momentum. But that's also a complicated issue (search for the Abraham-Minkowski paradox)

Could it possibly be explained by the very slight curvature in space created by atoms?

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Could it possibly be explained by the very slight curvature in space created by atoms?

No. Gravity is far too weak to explain such a phenomenon.

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No. Gravity is far too weak to explain such a phenomenon.

So if there's hardly any interaction or just no interaction, how is the light path distorted?

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No. Gravity is far too weak to explain such a phenomenon.

We are speculating what happens very close to particle. Something that happens in almost radius of proton, or electron.

force = mass / ( distance * distance ).

if distance between two particles is very small then any mass will be dominating over anything else.

mass=1, distance=1

force = 1/ (1*1) = 1

mass=10000, distance=100

force=10000/(100*100)=1

mass=1000000,distance=1000

force=1000000/(1000*1000)=1

Edited by Przemyslaw.Gruchala
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We are speculating what happens very close to particle. Something that happens in almost radius of proton, or electron.

No, we are discussing what established physics has to say.

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No, we are discussing what established physics has to say.

Which means nothing has to say, because Standard Model has no gravitation included.

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Which means nothing has to say, because Standard Model has no gravitation included.

Wrong thread. The standard model is not the topic here.

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So, to simplify the explanation to my question into something I can understand. Would I say, "Light is always traveling at 3*10^8, just its average forward motion is hindered, in some way (that being the interactions the photons have with the electrons), by whatever medium it's passing through"?

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So, to simplify the explanation to my question into something I can understand. Would I say, "Light is always traveling at 3*10^8, just its average forward motion is hindered, in some way (that being the interactions the photons have with the electrons), by whatever medium it's passing through"?

Photons are always traveling at this speed. Light slows down, because when you say "light" it is the average forward motion.

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• 3 weeks later...

Hi guys.I just felt according to maxwells relation of c is inversely proportional to permeability and permitivitty see glass has a higher dielectric contant than vacuum so that its relative permittivity is greater than in air so its speed is reduced.Light is nothing but mutually perpendicular oscillating electric and magnetic field,if the speed is reduced then it has to be something in the wave equation.

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