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Author Topic: Why does light slow down in a fiber optic cable?  (Read 6087 times)

Offline Geezer

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Why does light slow down in a fiber optic cable?
« on: 21/04/2010 04:47:43 »
I realize that the speed of light in glass is determined by the refractive index.

But why does light take longer to travel through glass than through space and which characteristic of glass determines that?


 

Offline JP

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Why does light slow down in a fiber optic cable?
« Reply #1 on: 21/04/2010 05:04:51 »
That's a tricky question.  The basics of it are that the light is interacting with the atomic structure of the glass in some way that slows it down.  The way it's often described is that the photons are absorbed and re-emitted by the atoms after a delay, but there's reasons to believe that's not the whole story (for one thing, atoms are very particular about the frequencies they tend to absorb/emit, and glass slows all visible light down, not just certain colors).  The explanation I've heard that makes more sense is that since all the glass atoms and molecules are bound together, the light interacts with the entire structure, and this interaction is what slows the light down. 

By the way, vibrations of the crystaline structure of the solid are called phonons: http://en.wikipedia.org/wiki/Phonon
I don't know how this applies to glass because it's amorphous (non-crystalline). 


By the way, since you asked about a fiber optic cable, there is also a net slow-down effect because the light is bouncing off the sides of the cable as it travels.  Only a part of the light's velocity will be directed along the cable's length, while a part of it is going into the light bouncing back and forth between the walls, so the total speed of the light is going to be slower than through a giant slab of glass, for example.
« Last Edit: 21/04/2010 05:07:25 by JP »
 

Offline L-Kira

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Why does light slow down in a fiber optic cable?
« Reply #2 on: 21/04/2010 05:29:10 »
this is because of total internal reflection and when Light travels through a glass medium light velocity decreases. Moreover,Velocity of Light can be reduced as light enters into any transparent medium as JP explains.

Furthermore, Dr.Michio Kaku will explain it for you

 

Offline syhprum

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Why does light slow down in a fiber optic cable?
« Reply #3 on: 21/04/2010 19:08:03 »
Do modern fibre optics cable in fact have multiple internal reflections I think the refractive index is adjusted across the diameter of the fibre to avoid this.
 

Offline lightarrow

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Why does light slow down in a fiber optic cable?
« Reply #4 on: 21/04/2010 20:26:29 »
I realize that the speed of light in glass is determined by the refractive index.

But why does light take longer to travel through glass than through space and which characteristic of glass determines that?
Remember that light is an electromagnetic field. A material is made of atoms which polarize in an em field; if this polarization were perfect, the field would be cancealed from the induced field and there would be no propagation at all (perfect conductor); since polarization is not perfect, the field does propagate, but with a different phase (with respect to the void): in a sense its presence in a specific point of space inside the material is retarded; then the process repeats with the subsequent layer of atoms and so on.
 

Offline Geezer

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Why does light slow down in a fiber optic cable?
« Reply #5 on: 21/04/2010 22:26:17 »
Remember that light is an electromagnetic field. A material is made of atoms which polarize in an em field; if this polarization were perfect, the field would be cancealed from the induced field and there would be no propagation at all (perfect conductor); since polarization is not perfect, the field does propagate, but with a different phase (with respect to the void): in a sense its presence in a specific point of space inside the material is retarded; then the process repeats with the subsequent layer of atoms and so on.

Thanks Lightarrow.

I was doing OK until I got to "different phase". I understand what phase means in general, but I don't get it in that context.
 

Offline LeeE

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Why does light slow down in a fiber optic cable?
« Reply #6 on: 21/04/2010 23:11:30 »
Do modern fibre optics cable in fact have multiple internal reflections I think the refractive index is adjusted across the diameter of the fibre to avoid this.

There are, broadly speaking, three methods used to contain the light within a fibre optic cable.  The important factors are the diameter of the cable, the refractive index gradient across the diameter of the cable and the wavelength of the light that the cable has been designed to carry.

The 'simplest' type of fibre optic cable has an internal diameter that is > ~10 times the wavelength of the light it is designed to convey and has a uniform refractive index across the diameter of the cable.  This type of cable can carry a relatively wide range of frequencies (wavelengths) and relies upon reflection at the boundary, where there is an abrupt change of refractive index i.e. between the outer surface of the cable and its 'cladding'.

Equally 'simple', at least in terms of construction, is the single-mode type of fibre cable, where the refractive index is also uniform across the diameter of the cable.  However, with single-mode fibres the diameter of the light channel is < ~10 times the wavelength of the light traveling though it and this means that instead of propagating in a geometrical manner down the cable (by reflecting off its boundaries) the light propagates more like EMR through shaped space-time  (However, in practice, most single-mode fibres are designed for IR wavelengths, so although the diameter of the fibre may be < ~10 wavelengths of the single-mode IR light it's been designed for, it also means that the diameter can be > ~10 time the wavelength of the blue end of optical light, so it's actually possible to use 'single-mode' fibre to convey two entirely separate channels simultaneously).

The most complex, in terms of design, is 'Graded Index Fibre'.  In this type of fibre the refractive index of the fibre increases with the radius across the area of the fibre, instead of being constant across the diameter of the fibre, so that instead of oblique light being reflected by the sharp refractive index change boundary, at the walls of the fibre, the light is more smoothly bent back towards the center of the light channel without actually being reflected at all.
 

Offline lightarrow

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Why does light slow down in a fiber optic cable?
« Reply #7 on: 26/04/2010 01:06:35 »
Remember that light is an electromagnetic field. A material is made of atoms which polarize in an em field; if this polarization were perfect, the field would be cancealed from the induced field and there would be no propagation at all (perfect conductor); since polarization is not perfect, the field does propagate, but with a different phase (with respect to the void): in a sense its presence in a specific point of space inside the material is retarded; then the process repeats with the subsequent layer of atoms and so on.

Thanks Lightarrow.

I was doing OK until I got to "different phase". I understand what phase means in general, but I don't get it in that context.
Because what changes inside glass is the "phase velocity" vp of the em wave:
vp = c/n.
 

Offline L-Kira

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Why does light slow down in a fiber optic cable?
« Reply #8 on: 30/04/2010 04:27:42 »
Remember that light is an electromagnetic field. A material is made of atoms which polarize in an em field; if this polarization were perfect, the field would be cancealed from the induced field and there would be no propagation at all (perfect conductor); since polarization is not perfect, the field does propagate, but with a different phase (with respect to the void): in a sense its presence in a specific point of space inside the material is retarded; then the process repeats with the subsequent layer of atoms and so on.

Thanks Lightarrow.

I was doing OK until I got to "different phase". I understand what phase means in general, but I don't get it in that context.
Because what changes inside glass is the "phase velocity" vp of the em wave:
vp = c/n.

there goes the light arrow. when lightarrow fires an arrow inside optical fiber. his arrow slows down due the the fact of "the delay factor". Because his arrow need to slow down when it passes each molecule of fiber optical cable.
 
guys did you know that invention of fiber optical cable was accidental while making special kind of glass!

L-Kira
« Last Edit: 30/04/2010 04:30:33 by L-Kira »
 

Offline lightarrow

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Why does light slow down in a fiber optic cable?
« Reply #9 on: 30/04/2010 12:10:37 »
Because what changes inside glass is the "phase velocity" vp of the em wave:
vp = c/n.
there goes the light arrow. when lightarrow fires an arrow inside optical fiber. his arrow slows down due the the fact of "the delay factor". Because his arrow need to slow down when it passes each molecule of fiber optical cable.
??? It was an ironic comment or what?
 

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Why does light slow down in a fiber optic cable?
« Reply #9 on: 30/04/2010 12:10:37 »

 

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