# Huygen's principle

• February 14th, 2008, 02:10 PM
scientist91
Huygen's principle
I want to know, when there is diffraction or refraction of light, are the atoms inside of the material releasing also waves in whole directions, like on this pictures:
Picture 1 and Picture 2
• February 14th, 2008, 02:28 PM
Cynapse
Picture 2 doesnt work for me.

But in picture 1 the yellow dots don't represent atoms. Where i think you've drawn in the yellow dots there is no material.

This I always find is a relatively confusing picture because it shows the grey lines but these are wave fronts, not the actual waves...[/b]
• February 14th, 2008, 02:37 PM
scientist91
Quote:

Originally Posted by Cynapse
Picture 2 doesnt work for me.

But in picture 1 the yellow dots don't represent atoms. Where i think you've drawn in the yellow dots there is no material.

This I always find is a relatively confusing picture because it shows the grey lines but these are wave fronts, not the actual waves...[/b]

And how refraction, works than? Why the wave move slowly in some mediums?
• February 14th, 2008, 03:06 PM
Cynapse
Quote:

Originally Posted by scientist91
Quote:

Originally Posted by Cynapse
Picture 2 doesnt work for me.

But in picture 1 the yellow dots don't represent atoms. Where i think you've drawn in the yellow dots there is no material.

This I always find is a relatively confusing picture because it shows the grey lines but these are wave fronts, not the actual waves...[/b]

And how refraction, works than? Why the wave move slowly in some mediums?

Refraction ie the change in angle, happens because the speed of light changes. The understanding is fairly simple and there are countless analogies on the web if you google Refraction.

The reason the speed changes however is a bit more complicated. Its roots lie in relativity. THE SPEED OF LIGHT IS CONSTANT EVERYWHERE. One interpretation is that essentially the light isn't slowing down at all (according to its own frame of reference), it is just taking longer than we in OUR frame of reference would expect for the length of glass or whatever. This means the light must be having to go further...
• February 14th, 2008, 03:10 PM
scientist91
Quote:

Originally Posted by Cynapse
Quote:

Originally Posted by scientist91
Quote:

Originally Posted by Cynapse
Picture 2 doesnt work for me.

But in picture 1 the yellow dots don't represent atoms. Where i think you've drawn in the yellow dots there is no material.

This I always find is a relatively confusing picture because it shows the grey lines but these are wave fronts, not the actual waves...[/b]

And how refraction, works than? Why the wave move slowly in some mediums?

Refraction ie the change in angle, happens because the speed of light changes. The understanding is fairly simple and there are countless analogies on the web if you google Refraction.

The reason the speed changes however is a bit more complicated. Its roots lie in relativity. THE SPEED OF LIGHT IS CONSTANT EVERYWHERE. One interpretation is that essentially the light isn't slowing down at all (according to its own frame of reference), it is just taking longer than we in OUR frame of reference would expect for the length of glass or whatever. This means the light must be having to go further...

If we would expect the photon everywhere, why the angle of incidence is equal to the angle of reflection?
• February 14th, 2008, 03:15 PM
Cynapse
Quote:

If we would expect the photon everywhere, why the angle of incidence is equal to the angle of reflection?
When did I say that??? I thought we were on about refraction?
• February 14th, 2008, 03:16 PM
KALSTER
Quote:

If we would expect the photon everywhere, why the angle of incidence is equal to the angle of reflection?
The reflected angle probability is represented by a sort of a bell-shaped curve, with the biggest probability being the same refected angle as the inceidence angle. So with a lot of photons hitting and reflecting, the vast majority will reflect with an angle equal to the incidence angle.
• February 14th, 2008, 03:18 PM
scientist91
Quote:

Originally Posted by KALSTER
Quote:

If we would expect the photon everywhere, why the angle of incidence is equal to the angle of reflection?
The reflected angle probability is represented by a sort of a bell-shaped curve, with the biggest probability being the same refected angle as the inceidence angle. So with a lot of photons hitting and reflecting, the vast majority will reflect with an angle equal to the incidence angle.

Wouldn't the photon "excite" the atoms inside of the material?
• February 14th, 2008, 03:24 PM
KALSTER
In the real world, yes. Most will just be reflected with no energy being passed to the reflective material, but some photons will be absorbed, adding energy to the reflective material which presents as heat mostly.
• February 14th, 2008, 03:29 PM
scientist91
Quote:

Originally Posted by KALSTER
In the real world, yes. Most will just be reflected with no energy being passed to the reflective material, but some photons will be absorbed, adding energy to the reflective material which presents as heat mostly.

Do the photon emits energy in whole directions, like on the hygens principle?
• February 14th, 2008, 03:34 PM
Cynapse
Quote:

Originally Posted by scientist91
Quote:

Originally Posted by KALSTER
Quote:

If we would expect the photon everywhere, why the angle of incidence is equal to the angle of reflection?
The reflected angle probability is represented by a sort of a bell-shaped curve, with the biggest probability being the same refected angle as the inceidence angle. So with a lot of photons hitting and reflecting, the vast majority will reflect with an angle equal to the incidence angle.

Wouldn't the photon "excite" the atoms inside of the material?

Not exactly. The photon excites the solid as a whole. When the atoms come together to make a solid their properties aren't strictly dictated by the individual atoms. The phonon spectrum (lattice vibrations) of the solid absorbs the photon and then is re-emmitted.
• February 14th, 2008, 03:38 PM
scientist91
Quote:

Originally Posted by Cynapse
Quote:

Originally Posted by scientist91
Quote:

Originally Posted by KALSTER
Quote:

If we would expect the photon everywhere, why the angle of incidence is equal to the angle of reflection?
The reflected angle probability is represented by a sort of a bell-shaped curve, with the biggest probability being the same refected angle as the inceidence angle. So with a lot of photons hitting and reflecting, the vast majority will reflect with an angle equal to the incidence angle.

Wouldn't the photon "excite" the atoms inside of the material?

Not exactly. The photon excites the solid as a whole. When the atoms come together to make a solid their properties aren't strictly dictated by the individual atoms. The phonon spectrum (lattice vibrations) of the solid absorbs the photon and then is re-emmitted.

Do the photon emits energy in whole directions, like on the hygens principle?
• February 14th, 2008, 03:45 PM
Cynapse
Photons don't emitt energy, they ARE energy. Huygens principal can't be applied to them because they aren't waves they are particles.
• February 14th, 2008, 03:52 PM
scientist91
Quote:

Originally Posted by Cynapse
Photons don't emitt energy, they ARE energy. Huygens principal can't be applied to them because they aren't waves they are particles.

I mean, do the electrons emit the energy in whole directions?
• February 14th, 2008, 03:55 PM
Cynapse
The electrons aren't really the thing thats reflecting the photons. Phonons are. These are the vibrations of the solid itself, rather than each individual atom. And yes they have a single direction when it is measured.
• February 14th, 2008, 03:59 PM
scientist91
Quote:

Originally Posted by Cynapse
The electrons aren't really the thing thats reflecting the photons. Phonons are. These are the vibrations of the solid itself, rather than each individual atom. And yes they have a single direction when it is measured.

It isn't like in the Huygen's principle?
• February 14th, 2008, 04:15 PM
scientist91
Cynapse?
• February 14th, 2008, 04:29 PM
KALSTER
Quote:

Originally Posted by scientist91
Cynapse?

I think he is busy pulling out his hair. Be patient.
• February 14th, 2008, 04:40 PM
Cynapse
Quote:

Originally Posted by KALSTER
Quote:

Originally Posted by scientist91
Cynapse?

I think he is busy pulling out his hair. Be patient.

LMFAO shhhh!

Look you cannot CANNOT CANNOT applu Hugens principal to a single photon. It is not a wave so wave methods cannot work. Im loosing track of what your asking now...
• February 14th, 2008, 04:42 PM
scientist91
Quote:

Originally Posted by Cynapse
Quote:

Originally Posted by KALSTER
Quote:

Originally Posted by scientist91
Cynapse?

I think he is busy pulling out his hair. Be patient.

LMFAO shhhh!

Look you cannot CANNOT CANNOT applu Hugens principal to a single photon. It is not a wave so wave methods cannot work. Im loosing track of what your asking now...

Look at this simulation you'll know what I am talking about.
Click 2 times on "Next Step"
• February 14th, 2008, 04:46 PM
Cynapse

The photon excites the solid as a whole. When the atoms come together to make a solid their properties aren't strictly dictated by the individual atoms. The phonon spectrum (lattice vibrations) of the solid absorbs the photon and then is re-emmitted.

Huygens principal cannot be used in anyway to explain the photon-phonon interraction as it is a quasi-particle interraction, not a wave-medium interraction. Forgetting the quantum aspect of it there are simplistically no waves whatsoever involved.
• February 14th, 2008, 04:48 PM
scientist91
Quote:

Originally Posted by Cynapse

The photon excites the solid as a whole. When the atoms come together to make a solid their properties aren't strictly dictated by the individual atoms. The phonon spectrum (lattice vibrations) of the solid absorbs the photon and then is re-emmitted.

Huygens principal cannot be used in anyway to explain the photon-phonon interraction as it is a quasi-particle interraction, not a wave-medium interraction. Forgetting the quantum aspect of it there are simplistically no waves whatsoever involved.

OK, i understand that there are phonons. But I want to know, is the light spread equally when it is absorbed? Why we cannot use the huygens principle? I found too much articles connecting huygens principle with reflection and reflection.
• February 14th, 2008, 04:53 PM
Cynapse
Huygens principal is used directly in demonstrating the reasons and explaining waves refracting and diffracting. Not directly in reflection.
• February 14th, 2008, 05:05 PM
scientist91
Quote:

Originally Posted by Cynapse
Huygens principal is used directly in demonstrating the reasons and explaining waves refracting and diffracting. Not directly in reflection.

Is the simulation correct?
• February 14th, 2008, 05:20 PM
Cynapse
Simulation of what?
• February 15th, 2008, 01:59 AM
scientist91
Quote:

Originally Posted by Cynapse
Simulation of what?

• February 15th, 2008, 09:32 AM
Cynapse
The diagram describes continuous waves. It is correct when treating light as a wave, not as individual photons.
• February 15th, 2008, 12:02 PM
scientist91
Quote:

Originally Posted by Cynapse
The diagram describes continuous waves. It is correct when treating light as a wave, not as individual photons.

Why the wave runs so slow in the 2-nd medium?
• February 15th, 2008, 01:43 PM
Cynapse
A solid has a network of ions and electrons fixed in a "lattice". Think of this as a network of balls connected to each other by springs. Because of this, they have what is known as "collective vibrational modes", often called phonons. These are quanta of lattice vibrations, similar to photons being the quanta of EM radiation. It is these vibrational modes that can absorb a photon. So when a photon encounters a solid, and it can interact with an available phonon mode (i.e. something similar to a resonance condition), this photon can be absorbed by the solid and then converted to heat (it is the energy of these vibrations or phonons that we commonly refer to as heat). The solid is then opaque to this particular photon (i.e. at that frequency). Now, unlike the atomic orbitals, the phonon spectrum can be broad and continuous over a large frequency range. That is why all materials have a "bandwidth" of transmission or absorption. The width here depends on how wide the phonon spectrum is.

On the other hand, if a photon has an energy beyond the phonon spectrum, then while it can still cause a disturbance of the lattice ions, the solid cannot sustain this vibration, because the phonon mode isn't available. This is similar to trying to oscillate something at a different frequency than the resonance frequency. So the lattice does not absorb this photon and it is re-emitted but with a very slight delay. This, naively, is the origin of the apparent slowdown of the light speed in the material. The emitted photon may encounter other lattice ions as it makes its way through the material and this accumulate the delay.

Moral of the story: the properties of a solid that we are familiar with have more to do with the "collective" behavior of a large number of atoms interacting with each other. In most cases, these do not reflect the properties of the individual, isolated atoms.

FROM

• February 15th, 2008, 01:46 PM
scientist91
Quote:

Originally Posted by Cynapse
A solid has a network of ions and electrons fixed in a "lattice". Think of this as a network of balls connected to each other by springs. Because of this, they have what is known as "collective vibrational modes", often called phonons. These are quanta of lattice vibrations, similar to photons being the quanta of EM radiation. It is these vibrational modes that can absorb a photon. So when a photon encounters a solid, and it can interact with an available phonon mode (i.e. something similar to a resonance condition), this photon can be absorbed by the solid and then converted to heat (it is the energy of these vibrations or phonons that we commonly refer to as heat). The solid is then opaque to this particular photon (i.e. at that frequency). Now, unlike the atomic orbitals, the phonon spectrum can be broad and continuous over a large frequency range. That is why all materials have a "bandwidth" of transmission or absorption. The width here depends on how wide the phonon spectrum is.

On the other hand, if a photon has an energy beyond the phonon spectrum, then while it can still cause a disturbance of the lattice ions, the solid cannot sustain this vibration, because the phonon mode isn't available. This is similar to trying to oscillate something at a different frequency than the resonance frequency. So the lattice does not absorb this photon and it is re-emitted but with a very slight delay. This, naively, is the origin of the apparent slowdown of the light speed in the material. The emitted photon may encounter other lattice ions as it makes its way through the material and this accumulate the delay.

Moral of the story: the properties of a solid that we are familiar with have more to do with the "collective" behavior of a large number of atoms interacting with each other. In most cases, these do not reflect the properties of the individual, isolated atoms.

FROM

Ok, My actual question, is do the atoms of the inside of the material, can release waves, like the atoms of the surface?
• February 15th, 2008, 02:10 PM
Cynapse
It is the lattice phonons absorb and re-emitt the energy. But yes the interraction between internal lattice ions will cause the photon to be absorbed into the phonon but then be re-emitted as a photon (which travels as a wave) due to the mode not being available.
• February 15th, 2008, 02:34 PM
scientist91
Quote:

Originally Posted by Cynapse
It is the lattice phonons absorb and re-emitt the energy. But yes the interraction between internal lattice ions will cause the photon to be absorbed into the phonon but then be re-emitted as a photon (which travels as a wave) due to the mode not being available.

And can you tell me concretely, why the light slows down in medium? That link that you gave me, is useful, but it is not actually concrete. Are the atoms of the internal structure also releasing waves so the refraction angle of the light is lower than the reflection light's angle?
• February 15th, 2008, 02:50 PM
Cynapse
Because of the repeated absorption and re-emission of photons by the phonons of the lattice. There is a slight delay each time.
• February 15th, 2008, 02:54 PM
scientist91
Quote:

Originally Posted by Cynapse
Because of the repeated absorption and re-emission of photons by the phonons of the lattice. There is a slight delay each time.

Are the waves real, or they are just way to describe the direction of the photon?
• February 15th, 2008, 02:57 PM
Cynapse
The photon-EM waves are real. The photon wave functions are probability functions.
• February 15th, 2008, 03:09 PM
scientist91
Quote:

Originally Posted by Cynapse
The photon-EM waves are real. The photon wave functions are probability functions.

And do the phonons from the internal structure release waves in up direction, like here?
• February 15th, 2008, 03:16 PM
Cynapse
They create a wave function in all directions shown by the circles (the WF wouldnt actually be circular but a different shape depending on the result but would cover all directions). But as soon as a photon is detected to have gone in one direction the wave function collapses, the circle disappears and an EM wave with a definite direction is detected.
• February 15th, 2008, 03:31 PM
scientist91
Quote:

Originally Posted by Cynapse
They create a wave function in all directions shown by the circles (the WF wouldnt actually be circular but a different shape depending on the result but would cover all directions). But as soon as a photon is detected to have gone in one direction the wave function collapses, the circle disappears and an EM wave with a definite direction is detected.

But if there is wave released of the internal structure, there will be also created light, like here? Do you understand, what I want to say?
• February 15th, 2008, 03:42 PM
Cynapse
I sort of see what you are trying to say (that the fact the waves are circuler can mean the go off in any direction?) but the WF probabilities and the destructive interference of the EM waves mean that whilst this may be possible, it will only possible to the order of >>> 10E-34 FOR EACH PHOTON..

That is :
>>> 0.00000000000000000000000000000000001% per photon

So to get a photon beam of say 10 photons this would give a probability of [10E-34E10]% which is 10E-[34*34*34*34*34*34*34*34*34*34]. To get a visible beam the number is so small it can be well approximated to 0%.
• February 15th, 2008, 03:48 PM
scientist91
Quote:

Originally Posted by Cynapse
I sort of see what you are trying to say (that the fact the waves are circuler can mean the go off in any direction?) but the WF probabilities and the destructive interference of the EM waves mean that whilst this may be possible, it will only possible to the order of >>> 10E-34 FOR EACH PHOTON..

That is :
>>> 0.00000000000000000000000000000000001% per photon

So to get a photon beam of say 10 photons this would give a probability of [10E-34E10]% which is 10E-[34*34*34*34*34*34*34*34*34*34]. To get a visible beam the number is so small it can be well approximated to 0%.

But we said that phonons of the internal material also are emitting waves in whole directions. If you said that it is not like that, why some atoms release, and some not release EM circular waves?
• February 15th, 2008, 04:01 PM
Cynapse
In this the phonons are absorbing the photons, not any individual atoms. [The phonons only absorb the photons due to interraction between photon and lattice ion.]

The only way an EM is released by the phonons is when they have absorbed a photon. They don't absorb a wave at different points on the circle. Only one photon at one place. NOT all along the circle. The circle just represents where the single photon can land if you like.

You are still trying to equate light as a wave AND particle. You are trying to merge the two diffent ways of looking at this which you fundamentally cant do.
• February 15th, 2008, 04:03 PM
scientist91
Quote:

Originally Posted by Cynapse
In this the phonons are absorbing the photons, not any individual atoms. [The phonons only absorb the photons due to interraction between photon and lattice ion.]

The only way an EM is released by the phonons is when they have absorbed a photon. They don't absorb a wave at different points on the circle. Only one photon at one place. NOT all along the circle. The circle just represents where the single photon can land if you like.

You are still trying to equate light as a wave AND particle. You are trying to merge the two diffent ways of looking at this which you fundamentally cant do.

Again, I will ask, do the circular waves, exist in reality?? Or they are just helpful for understanding the light?
• February 15th, 2008, 04:17 PM
Cynapse
The circle (ie Hugens principal) is a method of describing waves. The circle is describing how the wave is acting.

The concept of waves is confusing. How can you define a wave. The best definition I have heard was the method of transferring energy without transferring matter.

So the way in which the circles are used here is a beam of light. The circles then do exist. The only relavence they have to photons is the possible paths of each one. So if you want to view the problem light being a wave, forget photonic interraction and the quantum reason for light slowing down. Just treat the light as a wave which propgates in all directions. Then you take into account superposition and apply interference.
• February 15th, 2008, 05:37 PM
scientist91
Quote:

Originally Posted by Cynapse
The circle (ie Hugens principal) is a method of describing waves. The circle is describing how the wave is acting.

The concept of waves is confusing. How can you define a wave. The best definition I have heard was the method of transferring energy without transferring matter.

So the way in which the circles are used here is a beam of light. The circles then do exist. The only relavence they have to photons is the possible paths of each one. So if you want to view the problem light being a wave, forget photonic interraction and the quantum reason for light slowing down. Just treat the light as a wave which propgates in all directions. Then you take into account superposition and apply interference.

Quote:

is the elementary particle (specifically, a boson) responsible for electromagnetic phenomena. It is the carrier of electromagnetic radiation of all wavelengths, including gamma rays, X-rays, ultraviolet light, visible light, infrared light, microwaves, and radio waves. The photon differs from many other elementary particles, such as the electron and the quark, in that it has zero rest mass;[3] therefore, it travels (in a vacuum) at the speed of light, c. Like all quanta, the photon has both wave and particle properties (“wave–particle duality”). Photons show wave-like phenomena, such as refraction by a lens and destructive interference when reflected waves cancel each other out; however, as a particle, it can only interact with matter by transferring the amount of energy
So, as far as I understood, it is particle without mass, which is carrying electromagnetic field with itself.
• February 16th, 2008, 02:51 PM
bit4bit
Quote:

Originally Posted by Cynapse
Quote:

Originally Posted by scientist91
Quote:

Originally Posted by Cynapse
Picture 2 doesnt work for me.

But in picture 1 the yellow dots don't represent atoms. Where i think you've drawn in the yellow dots there is no material.

This I always find is a relatively confusing picture because it shows the grey lines but these are wave fronts, not the actual waves...[/b]

And how refraction, works than? Why the wave move slowly in some mediums?

Refraction ie the change in angle, happens because the speed of light changes. The understanding is fairly simple and there are countless analogies on the web if you google Refraction.

The reason the speed changes however is a bit more complicated. Its roots lie in relativity. THE SPEED OF LIGHT IS CONSTANT EVERYWHERE. One interpretation is that essentially the light isn't slowing down at all (according to its own frame of reference), it is just taking longer than we in OUR frame of reference would expect for the length of glass or whatever. This means the light must be having to go further...

I don't have a degree in physics, but I thought refaction was due to the electric permittivity and magnetic permeability of the medium being different to that of free space or air, due to the presence of charged particles (protons/electrons) within the molecular structure (even though there is no nett charge). I remember reading somewhere a while ago that the speed of light in a vacuum is given by an equation like:

c=1/ √(μ0*ε0)

Such that if μ0 and ε0 are changed, so will the speed of light. I thought that those values were at a minimum value for light in a vacuum, (and constant for free space) meaning that c cannot be increased within free space, but that they can increase within a material at the molecular level due to the presence of electric/magnetic fields on an atomic scale...decreasing the speed of light.

I haven't looked much at relativity, so I don't know for sure, but it seems to me that for light to take a longer path, it would have to be due to a curvature of spactime itself...and for the effect to be noticeable enough for refraction, a pretty large concentrated mass (With a strong gravitational field) would need to be present....If it were, then it would also affect the light outside of the refactive medium anyway.

I'm not sure about the reference frame idea, but it seems from what you've written, that this should also be the case OUTSIDE of the refactive medium too...meaning no refaction. I thought that the speed of light IN A VACUUM was constant (c), but in other cases like this, it can change due to small collective electric/magnetic effects of materials...although this only allows for a reduction in the speed of light, and not an increase.

I've always thought this was the case, but maybe I'm wrong? It seems more logical to me anyway.
• February 16th, 2008, 11:35 PM
Cynapse
• February 17th, 2008, 11:59 AM
bit4bit
You're right, my apologies...I didn't read the whole thread. I guess this post summed it up:

Quote:

Originally Posted by Cynapse
A solid has a network of ions and electrons fixed in a "lattice". Think of this as a network of balls connected to each other by springs. Because of this, they have what is known as "collective vibrational modes", often called phonons. These are quanta of lattice vibrations, similar to photons being the quanta of EM radiation. It is these vibrational modes that can absorb a photon. So when a photon encounters a solid, and it can interact with an available phonon mode (i.e. something similar to a resonance condition), this photon can be absorbed by the solid and then converted to heat (it is the energy of these vibrations or phonons that we commonly refer to as heat). The solid is then opaque to this particular photon (i.e. at that frequency). Now, unlike the atomic orbitals, the phonon spectrum can be broad and continuous over a large frequency range. That is why all materials have a "bandwidth" of transmission or absorption. The width here depends on how wide the phonon spectrum is.

On the other hand, if a photon has an energy beyond the phonon spectrum, then while it can still cause a disturbance of the lattice ions, the solid cannot sustain this vibration, because the phonon mode isn't available. This is similar to trying to oscillate something at a different frequency than the resonance frequency. So the lattice does not absorb this photon and it is re-emitted but with a very slight delay. This, naively, is the origin of the apparent slowdown of the light speed in the material. The emitted photon may encounter other lattice ions as it makes its way through the material and this accumulate the delay.

Moral of the story: the properties of a solid that we are familiar with have more to do with the "collective" behavior of a large number of atoms interacting with each other. In most cases, these do not reflect the properties of the individual, isolated atoms.

FROM