How light travels through glass

[bestpavan]

Hi,

I want to know how light travels through glass. As per my understanding of physics, a photon is absorbed by an electron and the electron goes to a high energy orbit and returns to its original state after a moment and emits another photon. This is how light travels in glass.

But how come the photons manage to go maintain their direction?

Thanks in advance!

 

[ansgar]

no glass is a solid and hence you must use the band description of energy levels:

 

[bestpavan]

Sorry! I didn't understand your reply! What about water? How does light travel in water? that too in straight lines?

 

[ansgar]

Sorry! I didn't understand your reply! What about water? How does light travel in water? that too in straight lines?

Photons from the visible part of the spectrum does not "travel through" glass in the way you proposed.

In water, visible light, is absorbed and re-emitted. This re-emission goes in all directions, this is why there is dark at the bottom of the ocean...

 

[bestpavan]

ohk.. let me put it this way... I have a glass of water.. and a photon enters it from the left side..and it is absorbed and is reemiited by the first atom in water.. and the process continues until it comes out from the right side of the water column.. why should this happen? And what is the problem with a piece of wood? why can't the atoms in it do the same thing?

 

[ansgar]

because of the energy-dependence of the absorption. Different material absorb at different wave-lengths.

and you should not confuse a photon with a light beam...

 

[bestpavan]

how is a light beam different from a group of photons? even a piece of wood takes a photon and reflects in back.. but doesn't transmit it.. why is that so? reflection also involves absorbtion and reemission right? but only in the same direction... why is that so?

 

[ansgar]

how is a light beam different from a group of photons? even a piece of wood takes a photon and reflects in back.. but doesn't transmit it.. why is that so? reflection also involves absorbtion and reemission right? but only in the same direction... why is that so?

you were saying "the photon"...

as for the wood and reflection, reflection is wavelength dependent as-well... that is why different objects have different colors.

 

[Edi]

I think the problem here is that you don't know/ understand that visible light is just a small part of the Electro Magnetic spectrum. Different frequency photons within the visible part have different colors. Frequencies outside the visible part just can't bee seen with our eyes. All the radio waves, microwaves, infra-red, visible, ultra-violet, x-ray and gamma rays are EM radiation with different frequencies .
Reflection and transparency is frequency-dependent. Different materials, different frequency.
Now, glass may be transparent to the small visible part of the spectrum, but at the same time may be reflective or absorptive to, say, infrared frequencies. A piece of wood may be reflective/ absorptive to visible part, but transparent to radio waves or... x-rays, i am not sure.

 

[ansgar]

... and also that these processes are probabilistic

 

[Schrodinger's Dog]

Hi,

I want to know how light travels through glass. As per my understanding of physics, a photon is absorbed by an electron and the electron goes to a high energy orbit and returns to its original state after a moment and emits another photon. This is how light travels in glass.

But how come the photons manage to go maintain their direction?

Thanks in advance!

Do you know about Snell's Law?http://en.wikipedia.org/wiki/Snell's_law?

It might also be an idea to read the FAQ here?

https://www.physicsforums.com/showthread.php?t=104715

Do Photons Move Slower in a Solid Medium?_{Do Photons Move Slower in a Solid Medium?}

Contributed by ZapperZ. Edited and corrected by Gokul43201 and inha

This question appears often because it has been shown that in a normal, dispersive solid such as glass, the speed of light is slower than it is in vacuum. This FAQ will strictly deal with that scenario only and will not address light transport in anomalous medium, atomic vapor, metals, etc., and will only consider light within the visible range.

The process of describing light transport via the quantum mechanical description isn't trivial. The use of photons to explain such process involves the understanding of not just the properties of photons, but also the quantum mechanical properties of the material itself (something one learns in Solid State Physics). So this explanation will attempt to only provide a very general and rough idea of the process.

A common explanation that has been provided is that a photon moving through the material still moves at the speed of c, but when it encounters the atom of the material, it is absorbed by the atom via an atomic transition. After a very slight delay, a photon is then re-emitted. This explanation is incorrect and inconsistent with empirical observations. If this is what actually occurs, then the absorption spectrum will be discrete because atoms have only discrete energy states. Yet, in glass for example, we see almost the whole visible spectrum being transmitted with no discrete disruption in the measured speed. In fact, the index of refraction (which reflects the speed of light through that medium) varies continuously, rather than abruptly, with the frequency of light.

Secondly, if that assertion is true, then the index of refraction would ONLY depend on the type of atom in the material, and nothing else, since the atom is responsible for the absorption of the photon. Again, if this is true, then we see a problem when we apply this to carbon, let's say. The index of refraction of graphite and diamond are different from each other. Yet, both are made up of carbon atoms. In fact, if we look at graphite alone, the index of refraction is different along different crystal directions. Obviously, materials with identical atoms can have different index of refraction. So it points to the evidence that it may have nothing to do with an "atomic transition".

When atoms and molecules form a solid, they start to lose most of their individual identity and form a "collective behavior" with other atoms. It is as the result of this collective behavior that one obtains a metal, insulator, semiconductor, etc. Almost all of the properties of solids that we are familiar with are the results of the collective properties of the solid as a whole, not the properties of the individual atoms. The same applies to how a photon moves through a solid.

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.

 

[bestpavan]

Thanks for this post!

I think 50% of my question has been resolved. However, I have this question now.

Howcome an x-ray photon can pass through the human flesh without any issue but a visible light,say blue photon is reflected back?

 

[Schrodinger's Dog]

Thanks for this post!

I think 50% of my question has been resolved. However, I have this question now.

Howcome an x-ray photon can pass through the human flesh without any issue but a visible light,say blue photon is reflected back?

Have you heard of statistical waves.

Ie group and phase velocity and ?

Cheronkov radiation.

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

Nothing can exceed c not even gamma rays or photons which are made up of them.

 

[lightarrow]

but still.. how is Cherenkov radiation related to my question?
"Howcome an x-ray photon can pass through the human flesh without any issue but a visible light,say blue photon is reflected back?"

Because X-ray photons have a very large momentum, in comparison to the visible light ones. It's as if you shooted ping-pong balls against concrete (visible light photons) or lead bullets against butter (X-ray photons).
Furthermore, visible light finds easily chemicals which absorbs it, in the human flesh (actually, in the skin already) because there are many atoms and compounds with electronics energy levels in that range of EM radiation, while for X-rays that's much less probable.

 

[Schrodinger's Dog]

Because X-ray photons have a very large momentum, in comparison to the visible light ones. It's as if you shooted ping-pong balls against concrete (visible light photons) or lead bullets against butter (X-ray photons).

Precisely.

 

[bestpavan]

I didn't really understand the bullet anology... How come a blue photon cannot pass through a less denser milk but can pass through a more dense glass??

 

[Edi]

Because it matters what the material is made of... (?)

 

[Schrodinger's Dog]

Everything is clear without the uncertainty principle. Photon is a wave. It travels in a wavy trajectory between atoms. Because of friction though its velocity decreases.

Indeed.

Superposition is clear only when you consider particle/wave duality otherwise it makes no sense.

If a tree falls in the forest and no one is around to hear it does it make a sound?

Thus define hear and sound?

Are they different or not?

Is a qualia an experience that you can sum to maths?

Is red for me the same as for you?

 

[Edi]

Indeed.


Is red for me the same as for you?

Now THAT is a freekin good question...

 

[lightarrow]

I didn't really understand the bullet anology... How come a blue photon cannot pass through a less denser milk but can pass through a more dense glass??

Because of what I wrote in the second part of my post...
When you treat light-matter interaction you have to specify what kind of interaction you want to analyze, or the answers would be quite generic. For a photon in the visible range interacting with milk or glass, the main interaction is with the electronic energy levels of the atoms and/or with the phonons of the entire structure (as in the case of glass), so in this case it's not the photon momentum that counts, but its frequency, how that matches those energy levels. If instead the interaction is mainly due to photon/electron scattering (e.g. Compton scattering), then what counts is the photon's momentum.

 

[conway]

I think lightarrow may be wrong about matching energy levels with phonons. I'm pretty sure photons interact with lattice vibrations ("phonons" as people call them) when the momentum matches up, not when the energy matches. In that respect it's similar to Compton scattering, and for similar reasons. When the momentum matches, the wavelength matches. And for a lattice vibration, if there is any electric polarization associated with the mechanical deformations of the material, you create planes of charge at precisely the wavelength of the incident light, give the light wave something to "bite" into. Hence the interaction.

 

[cesiumfrog]

how come the photons manage to go maintain their direction?

They are found in the direction where the wave-interference is constructive.

How come a blue photon cannot pass through a less denser milk but can pass through a more dense glass??

Milk is a bad example: Its colour and opacity has to do with larger scale structure and internal reflection rather than intrinsic absorptivity, like comparing clouds to rainwater (or dry paper to oily paper).