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  #1  
Old 18-05-2011, 03:55 PM
Keshdogga (Casey)
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Why does light refract?

Hey all I've been thinking lately, why does light actually refract. When I've asked this I get answers like "because the refractive indecies are different" or "because light travels slower through differnet mediums" but this does not answer my question.

What I was wondering was what properties of light and the molecules of the medium cause light to bend as it 'passes through'. Does 'c' actually decrease in medium just because there is more molecules??? The spaces between molecules is, in essence, a vacuum (neglecting virtual particles) so the speed of light shouldn't change just because density increases!! DOES NOT COMPUTE.

Is there some quantum effect that compensates for this?

Much appreciated,
Casey

PS. I also realise this may be a common topic of discussion on this board so sorry if i'm repeating the question
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  #2  
Old 18-05-2011, 04:09 PM
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renormalised (Carl)
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Quote:
"because the refractive indecies are different" or "because light travels slower through differnet mediums" but this does not answer my question.

What I was wondering was what properties of light and the molecules of the medium cause light to bend as it 'passes through'. Does 'c' actually decrease in medium just because there is more molecules???
In essence, with what I quoted from your post above you've basically answered your own question.

Because the light is passing through a material of higher density than a vacuum, the velocity of the light changes (photons interacting with the particles in the medium) and the direction in which it travels changes as a consequence. The higher the density of the material and the higher the R.I., the greater the change in direction of travel. Once the light reaches the other boundary between the material and the vacuum, the light resumes its original path and velocity. You have to remember, photons have a duel particle-wave nature and in the case of refraction/reflection, they are behaving as waves.

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

http://en.wikipedia.org/wiki/Snell%27s_law

I would suggest that you find and buy a high school textbook on physics and look up the section on light and its properties. Then try the problems in the book and see how you go.

Last edited by renormalised; 18-05-2011 at 04:33 PM.
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  #3  
Old 18-05-2011, 04:14 PM
adman (Adam)
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its more about the transition from one medium to the other. One analogy that can be useful is driving a car from hard tarmac onto sand at an angle. One wheel hits the sand first and is slowed down, so the car pulls to that side, then when all the wheels are on the sand it continues straight...

does that make any sense? It helps to imagine the light as a wavefront, rather than as a particle.
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  #4  
Old 18-05-2011, 04:53 PM
Keshdogga (Casey)
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Yes guys thanks for the responces =] But I'm still curious as to what the interaction is between light and the particles of the medium that causes it to bend at a predictable angle.

So we can obviously agree that the medium does not effect the speed of the light, light still always travels at c, it's only its apparent decrease in velocity. But when i try to imagine why the photon bends like a car turns one direction when it hits water i hit a dead end. the car does this because one side is going slower than the other not because the whole car has slowed and turned. Maybe this isn't an accurate example or i'm interpreting it wrong??

This is my unfulfilled understanding with the gaps

So the photon (singular) interacts with the molecules (i still don't know how (absorbed by electrons and re-emitted giving the appearence of 'bouncing' from atom to atom??)) in such a way that it appears to bend predictably. Should the light not just scatter if this is true?
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  #5  
Old 18-05-2011, 04:59 PM
Keshdogga (Casey)
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Or... is it in the transition between the mediums which causes the initial 'bend'? Like, because light moves in a wave does the trough and crest of the wave give me the left and right wheels of my car??
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  #6  
Old 18-05-2011, 05:27 PM
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This is way out of my field but I think you need to forget about photons hitting a diferent media and look at it as an electromagnetic radiation (wavefront) that has its phase delayed (or leads) as it enters the new media this gives a change in direction but not a change in speed.

Barry
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  #7  
Old 18-05-2011, 05:38 PM
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renormalised (Carl)
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Quote:
Originally Posted by Keshdogga View Post
Yes guys thanks for the responces =] But I'm still curious as to what the interaction is between light and the particles of the medium that causes it to bend at a predictable angle.

So we can obviously agree that the medium does not effect the speed of the light, light still always travels at c, it's only its apparent decrease in velocity. But when i try to imagine why the photon bends like a car turns one direction when it hits water i hit a dead end. the car does this because one side is going slower than the other not because the whole car has slowed and turned. Maybe this isn't an accurate example or i'm interpreting it wrong??

This is my unfulfilled understanding with the gaps

So the photon (singular) interacts with the molecules (i still don't know how (absorbed by electrons and re-emitted giving the appearence of 'bouncing' from atom to atom??)) in such a way that it appears to bend predictably. Should the light not just scatter if this is true?
The light doesn't act as a particle, it acts as a wave. The velocity of the light does, in reality, decrease as it enters the denser medium. It's not an apparent decrease. The direction of travel changes because the wave changes its velocity upon entering the denser medium. Those parts of the wave still outside the medium remain at their original velocity until they enter the medium. You answered your question in the bold analogy, above. You have to get used to thinking along the lines of the particle-wave duality. A photon is both simultaneously and it's acting as a wave when it enters the interface between the two media.

Light also scatters and does act as particles as well

You have two types of scattering/reflection, Specular and Lambertian. Specular scattering/reflection occurs when light strikes a surface and then scatters (reflects) back at the same angle it strikes that surface, much like a mirror. Lambertian scattering (reflection) is where the incident light strikes a surface and scatters equally in all directions, no matter the angle of incidence. Calm water is a specular scattering surface and the reason why it acts like a mirror when you look into it. An example of a Lambertian scattering surface would be sandpaper....basically any rough textured surface will scatter light in all directions, no matter the incident angle of the incoming rays (this being within reason...a mountain range is not a Lambertian reflection surface!!!).
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  #8  
Old 18-05-2011, 06:13 PM
takdriver (Graeme)
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light

Because when light enters a new substance, its speed changes. In order for the law demonstrated in the equation (frequency = speed/wavelength) to remain the same, the wavelength of the light particle/wave must shorten or lengthen. This change causes the wave to slightly refract to one side or the other.
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  #9  
Old 19-05-2011, 09:45 AM
Keshdogga (Casey)
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Quote:
Originally Posted by renormalised View Post
The light doesn't act as a particle, it acts as a wave. The velocity of the light does, in reality, decrease as it enters the denser medium. It's not an apparent decrease. The direction of travel changes because the wave changes its velocity upon entering the denser medium. Those parts of the wave still outside the medium remain at their original velocity until they enter the medium. You answered your question in the bold analogy, above. You have to get used to thinking along the lines of the particle-wave duality. A photon is both simultaneously and it's acting as a wave when it enters the interface between the two media.

Light also scatters and does act as particles as well

You have two types of scattering/reflection, Specular and Lambertian. Specular scattering/reflection occurs when light strikes a surface and then scatters (reflects) back at the same angle it strikes that surface, much like a mirror. Lambertian scattering (reflection) is where the incident light strikes a surface and scatters equally in all directions, no matter the angle of incidence. Calm water is a specular scattering surface and the reason why it acts like a mirror when you look into it. An example of a Lambertian scattering surface would be sandpaper....basically any rough textured surface will scatter light in all directions, no matter the incident angle of the incoming rays (this being within reason...a mountain range is not a Lambertian reflection surface!!!).
Thanks for you're responce Carl but unfortunately i can't get much more out of what you're saying other than light slows down when it hits a denser mdeium because it does. I'm still boggled as to how density can cause the wavelength of light to change? Doesn't the wavelength of a photon only change its energy? Am I being ignorant of some fundamental properties of light?
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  #10  
Old 19-05-2011, 09:52 AM
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Thanks for you're responce Carl but unfortunately i can't get much more out of what you're saying other than light slows down when it hits a denser mdeium because it does. I'm still boggled as to how density can cause the wavelength of light to change?
You can walk around a pool easily enough....how much harder is it to dive in the pool and walk across it?? It's exactly the same principle. The light finds it harder to move through the denser medium, its velocity of motion changes (slows down) and because it slows down the vector motion of the wave of light changes. Same as your car hitting the pool of water. The moment of inertia of one side of the car changes in value from the free side and the car spins. It's the same with the wavefront of the light passing into the denser medium.

Last edited by renormalised; 19-05-2011 at 10:07 AM.
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  #11  
Old 19-05-2011, 05:34 PM
adman (Adam)
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with sound it can go the other way - a more dense medium can transmit sound more easily, and the sound will travel faster.

The speed of light is a constant - but only in a vaccuum. Like the speed of sound is only 340 m/s in dry air at 20 degrees C. It varies with the medium.

I do see you point though - it would be nice to know the exact nature of the interaction with the medium that causes the change in speed. With sound it is almost self explanatory - with light - less so.
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  #12  
Old 19-05-2011, 06:33 PM
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refract etc

IN A VACUUM same, same. When light hits a medium the different wavelengths behave differently. ie. when light hits a prism. Check it out.:screwy :
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  #13  
Old 19-05-2011, 06:45 PM
Keshdogga (Casey)
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With sound it is almost self explanatory - with light - less so.
This is what i was streesing over I just can't wrap my head around this concept of light 'slowing down' so does this then mean light can have a speed of anywhere between 0m/s and c? or rather 0 < light's velocity ≤ c ? More of a clarification issue at the moment I think.

I was reading up on it more and found the wiki page on snell's law carl recomended (http://en.wikipedia.org/wiki/Snell%27s_law). Under the derivations and formule heading it says light takes the path of least time. This makes a bit more sense lol but I still can't explain why light takes the path of least time in the first place. Again It's hard to comprehend "why?" in terms of everyday examples because light is well... unique. Can anyone explain this to me?

Oh and the reason i've been so persistant with this question is just because i have a uni assignment on refractive indices and i hit a gap so to speak

Much appreciated,
Casey
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  #14  
Old 19-05-2011, 07:01 PM
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The simplest way to think of what is happening is when light which is a self sustaining electromagnetic wave enters a medium made up of atoms. The electrons of those atoms will oscillate and re-radiate at the same frequency. This will change the resulting EM wave vector. The positive nuclei are too massive to be affected.

Ordered crystals can have properties such as birefringence or polarisation. This is due to the non isotropic arrangement of atoms and hence electrons.

Cubic lattices of course are isotropic. Such as Fluorite. Glass is isotropic due to its random structure.

Metals attenuate light because of the free electrons.

Bert
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  #15  
Old 19-05-2011, 07:07 PM
sarman
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vol

In a nutshell, Lightwaves follow the path of least resistance. Sarman.
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  #16  
Old 19-05-2011, 08:40 PM
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Casey,

I think I understand where you're coming from.
This is my viewpoint on the physics involved. Consider light travelling through a denser medium such as glass.

The explanation is based (as Carl mentioned earlier) on both the wave and particle nature (or packet energy) of the light. The atomic structure of glass is such that the packet energy of any photon is absorbed and then re-emitted by constituent atoms rather than being absorbed and held as heat as in opaque materials. Glass atoms will absorb and then re-emit this packet energy in an essentially random chain throughout the glass but in the general direction of the refracted wavefront. It is important to note that any individual photon is reconverted many many times from one energy packet to another from atom to atom through the glass.
Exit photons are not the same ones that entered the glass. Due to the quantum nature of these energy conversions, it is impossible to trace the "path" of an individual photon packet.
The reason light slows down in glass is because of the time lag in passing this packet of energy from atom to atom. The initial bending (refraction) is due to the slowdown in energy transfer of photons in the wavefront nearer the glass compared to the photons still travelling through the air in the same wavefront further from the glass.

Regards, Rob
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  #17  
Old 19-05-2011, 10:37 PM
Keshdogga (Casey)
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Thank you Rob =D I totally understand now. You explained it perfectly. I was just unsure how light travelled through a denser medium. Also thank's for explaining why a light beam doesn't completely scatter on impact with the medium i had a feeling it may have had something to do with the initial direction.

And can i can get rid of this whole concept of light travelling 'slower' and just imagine a photon (on average) taking longer to pass through the medium because it is absorbed and emitted with a time lapse? Denser material, more absorbtion and emitting per cubed unit, apparent decrease in velocity?

Thanks Rob and to everyone else =]
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Old 19-05-2011, 10:42 PM
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Sorry, not on topic,
Where is CraigS????
Just curious.......
Bartman
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  #19  
Old 19-05-2011, 11:41 PM
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renormalised (Carl)
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Quote:
Originally Posted by Keshdogga View Post
Thank you Rob =D I totally understand now. You explained it perfectly. I was just unsure how light travelled through a denser medium. Also thank's for explaining why a light beam doesn't completely scatter on impact with the medium i had a feeling it may have had something to do with the initial direction.

And can i can get rid of this whole concept of light travelling 'slower' and just imagine a photon (on average) taking longer to pass through the medium because it is absorbed and emitted with a time lapse? Denser material, more absorbtion and emitting per cubed unit, apparent decrease in velocity?

Thanks Rob and to everyone else =]
You can't get rid of the concept of light traveling slower though a denser medium...it does travel slower, it's not an apparent change in velocity. It's a real difference in velocity. Go and have a look at these...

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

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

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


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Old 20-05-2011, 07:00 AM
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And just to complicate the issue further under certain circumstances the phase velocity of light can actually exceed c.

http://en.wikipedia.org/wiki/Refract...Speed_of_light

Regards

Steven
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