How is light absorbed/reflected?

  • Thread starter webdesignmad
  • Start date
  • Tags
    Light
In summary, the interaction between an atom and light is described by two terms in Hamilton's equations. The linear term is responsible for emission and absorption processes, while the quadratic term is responsible for scattering processes. These processes don't conserve energy and are very unlikely to occur.
  • #1
webdesignmad
3
0
Light particles, or photons, are "created" when an electron moves down 1 or more orbitals and the energy of the photon equals the energy difference between the rings. Light is then absorbed by an electron receiving the energy and moving up the proportional amount of orbitals. But electrons always return to their original orbitals by releasing this gained energy, so how then can light disappear, as the cycle starts again?
 
Physics news on Phys.org
  • #2
Excitation and deexcitation of atoms is only one small way that light interacts with matter, corresponding for example to the spectral lines in solar radiation. Most of the interaction is in the form of scattering and thermal motion.
 
  • #3
but scattering is absorption and random re-emtition no?
You can look at it classically with Maxwell's equations and look at the refractive index on two sides of an interface of say air and metal, but on a microscopic level, quantum i thought light scatters by absorption and reemition
 
  • #4
aimforclarity said:
but scattering is absorption and random re-emtition no?

Not really. Reemission usually takes some time, while scattering is usually considered instantaneous. Therefore real absorption and reemission is associated with fluorescence. However, elastic or inelastic scattering can be described by absorption to and reemission from a virtual energy state.
 
  • #5
So is light scattered by radiation?

What I'm asking is that, for example, if I turn off a light, the photons are immediately dispersed. Where have they gone? Or what has destroyed the quanta of energy?
 
  • #6
most of the photons are converted into vibrations of the molecules that absorbed the photons.
 
  • #7
Thanks, that's just what i was looking for
 
  • #8
Cthugha said:
Not really. Reemission usually takes some time, while scattering is usually considered instantaneous. Therefore real absorption and reemission is associated with fluorescence. However, elastic or inelastic scattering can be described by absorption to and reemission from a virtual energy state.

Can you give an example of instantaneous scattering ?
Every reference I have seen defines the mechanism of scattering , as well as reflection and refraction , as absorption / reemission
 
  • #9
An example? Pretty much any instance of scattering works that way. If scattering is described in terms of absorption and reemission, these are virtual processes and therefore also "instantaneous" compared to spontaneous reemission from a real state.

There is a review article called "Laser Rayleigh scattering" by R. B. Miles et. al. (Meas. Sci. Technol. 12 (2001) R33–R51) in case you are interested in that topic in depth. It should be freely available, if I remember correctly.
 
  • #10
Cthugha said:
An example? Pretty much any instance of scattering works that way. If scattering is described in terms of absorption and reemission, these are virtual processes and therefore also "instantaneous" compared to spontaneous reemission from a real state.

There is a review article called "Laser Rayleigh scattering" by R. B. Miles et. al. (Meas. Sci. Technol. 12 (2001) R33–R51) in case you are interested in that topic in depth. It should be freely available, if I remember correctly.

to be honest I am very confused now, is there any way you can describe the process to someone unfamilar with virtual photons but in a physics phd program
 
  • #11
This is somewhat hard without actually doing the math, so I suggest you pick up a book where the interaction between an atom and light is discussed in detail. One understandable approach is given e.g. in the introduction to quantum optics book by Grynberg, Aspect, Fabre and Cohen-Tannoudji.

In a nutshell you find that the interaction Hamiltonian in the long-wavelength approximation has two terms. One is linear and one is quadratic in the vector potential. The linear term contains one operator acting on the light field and one acting on the atom and is responsible for emission and absorption processes. The quadratic term contains two operators acting on photons, but none acting on the atom. You have four possible combinations of two operators acting on photons. One is creating two photons. One is destroying two photons. These processes do not conserve energy and are very unlikely to occur. The other two processes destroy one photon and create one photon without affecting the atom at all. These are the processes leading to elastic scattering.
 
  • #12
ok that makes a bit more sense Cthugha, thank you.

so there are 2 scattering processes for an atom in free space:
1. the linear abs/emit
2. quadratic a, a-dagger combinations for the light field that emit a photon of the same energy but in a random direction? (should be quasi-random right, only on average would it appear random)

my followup thoughts would be:
A. phase relation b/w incident & scattered photon -> 'coherent?'

B. does the incident photon 'slow down' or how much 'time' does this process take

C. the anhialated photon is the incident one and the scattered one is the outgoing one, so where is the virtual one? (sorry about my lack of qed understanding)

D. just to be clear when you say long wavelength limit that means semiclassical derivation - eg photons are not quantized? or what is the importance?

E. but this is not the same process that would occur for light scattering in a piece of plastic or a crystalline structure?

sorry for all the question, i really hope you could shed some light on my ignorance :)
 
Last edited:
  • #13
aimforclarity said:
A. phase relation b/w incident & scattered photon -> 'coherent?'

Elastic scattering with a single scatterer is usually coherent, yes.

aimforclarity said:
B. does the incident photon 'slow down' or how much 'time' does this process take

Well, the incoming wave or photon can experience a phase shift which could be interpreted as causing a tiny retardation. Apart from that, there is no slowing down.

aimforclarity said:
C. the anhialated photon is the incident one and the scattered one is the outgoing one, so where is the virtual one? (sorry about my lack of qed understanding)

There are no virtual photons, but virtual energy states involved. The annihilated photon should in principle cause some change before the other photon gets emitted in order to conserve energy. However, there are no allowed states to go to. However, due to uncertainty reasons, energy levels are not defined very well at short time intervals and the annihilated photon is modeled as going to an intermediate virtual state which is extremely short lived and therefore its energy is not well defined. Therefore you conserve energy.

aimforclarity said:
D. just to be clear when you say long wavelength limit that means semiclassical derivation - eg photons are not quantized? or what is the importance?

The long wavelength limit just means that the wavelength is so large compared to the size of the scattering particle that the shape of that particle does not matter. If the wavelength gets shorter, the actual shape does matter. That basically is the difference between Rayleigh scattering and Mie scattering.

aimforclarity said:
E. but this is not the same process that would occur for light scattering in a piece of plastic or a crystalline structure?

In crystalline structures you rarely have scattering from a single scatterer like an atom, but rather a large number of scatterers which may be more (ions) or less (electrons in metals) at well defined positions
 
  • #14
Cthugha said:
Elastic scattering with a single scatterer is usually coherent, yes.
-usually? maybe you can be a bit more explicit about what you mean by coherence? i was imagining that the phase and polarization of the outgoing photon is the same as the ingoing one
Cthugha said:
There are no virtual photons, but virtual energy states involved. The annihilated photon should in principle cause some change before the other photon gets emitted in order to conserve energy. However, there are no allowed states to go to. However, due to uncertainty reasons, energy levels are not defined very well at short time intervals and the annihilated photon is modeled as going to an intermediate virtual state which is extremely short lived and therefore its energy is not well defined. Therefore you conserve energy.

- this makes sense to me if i believe the that for processes on short time scales the effective "linewidth" of the energy levels grow. but this seems strange to me?


i also found this interesting 'physical' explanation byBorn2bwire of photon scattering in terms of more classical concepts and the idea of fields, i think he is talking about the same process you are

Born2bwire said:
As for the scattering, that arises due to the coupling of the photon's fields with the electron cloud of the bulk's atoms. The electromagnetic field from the photon disturbs the electron cloud which in turn creates its own electromagnetic wave in response. You can think of the total sum of these waves as giving rise to the scattering of the photon. In this manner, you do not need to be at an absorption level to achieve scattering. I believe that you can look into Griffith's introductory quantum mechanics books and find a few sections regarding scattering.
 
  • #15
aimforclarity said:
-usually? maybe you can be a bit more explicit about what you mean by coherence? i was imagining that the phase and polarization of the outgoing photon is the same as the ingoing one

Not necessarily the same phase, but at least some well defined relationship between initial and outgoing phase. Introduction of a well defined phase shift still counts as coherent.

The "usually" was aiming at the small probability of having inelastic scattering. For molecules, there is for example some small probability for scattering from an excited state of the molecule instead of from the ground state. So the initial and the final state of the molecule differ and the energy of the scattered photon is different from the incoming photon. This spontaneous scattering event (spontaneous Raman scattering) is inelastic and incoherent.

aimforclarity said:
- this makes sense to me if i believe the that for processes on short time scales the effective "linewidth" of the energy levels grow. but this seems strange to me?

It is strange, but in accordance with experiments.

aimforclarity said:
i also found this interesting 'physical' explanation byBorn2bwire of photon scattering in terms of more classical concepts and the idea of fields, i think he is talking about the same process you are

Yes, it is quite possible that he means the same situation.
 
  • #16
Cthugha said:
The "usually" was aiming at the small probability of having inelastic scattering. For molecules, there is for example some small probability for scattering from an excited state of the molecule instead of from the ground state. So the initial and the final state of the molecule differ and the energy of the scattered photon is different from the incoming photon. This spontaneous scattering event (spontaneous Raman scattering) is inelastic and incoherent.
ok! so if you have a linewidth on your state (eg a non-ground state) then you can also `transition` within the linewidth with some small probabily, which would be incoherent and inelastic. the reason this is called spontaneous Raman scattering is because it only occurs sometimes?
 
Last edited:
  • #17
It is not necessarily within the linewidth. Mostly it is assumed to be some excited state, say the lowest vibrational excitation of the molecule.

It is called spontaneous as opposed to stimulated. The meaning is the same as in spontaneous versus stimulated emission. The emission or scattering rate to the vacuum state is given by the spontaneous rate, but if the final state is already populated with photons, the emission/scattering rate gets enhanced proportionally. These are stimulated processes.
 

What is light and how is it absorbed?

Light is a form of electromagnetic radiation that is visible to the human eye. It is made up of particles called photons. When light hits an object, it can be either absorbed, reflected, or transmitted. Absorption occurs when the photons are absorbed by the atoms and molecules in the object.

What determines the color of an object?

The color of an object is determined by the wavelengths of light that it reflects. Objects that appear red reflect longer wavelengths of light, while objects that appear blue reflect shorter wavelengths. The color of an object can also be affected by the materials it is made of and the way the light is absorbed and scattered.

How is light absorbed by different materials?

The way light is absorbed by different materials depends on their chemical composition and physical structure. Some materials, such as metals, have free electrons that can easily absorb and re-emit light. Other materials, such as glass, have ordered atomic structures that allow them to absorb and transmit light in specific ways.

Why do objects appear different colors in different types of light?

Objects can appear different colors in different types of light because of the different wavelengths of light present in each type of light. For example, objects may appear different under natural sunlight compared to artificial indoor lighting. This is because sunlight contains the full spectrum of visible light, while artificial lighting may have a limited range of wavelengths.

How is light reflected by different surfaces?

Light can be reflected by different surfaces in different ways. Smooth and shiny surfaces, like mirrors, tend to reflect light in a regular and predictable manner. Rough and matte surfaces, on the other hand, tend to scatter light in all directions. The color and texture of a surface can also affect the way light is reflected.

Similar threads

  • Quantum Physics
Replies
4
Views
916
Replies
1
Views
776
  • Quantum Physics
Replies
4
Views
2K
  • Quantum Physics
2
Replies
36
Views
1K
  • Quantum Physics
Replies
3
Views
1K
  • Quantum Physics
Replies
1
Views
662
  • Quantum Physics
2
Replies
38
Views
3K
Replies
18
Views
1K
Back
Top