A mirror reflects light. The phenomenon of reflection involves momentary absorption and release of light from electron or atom.Due to this, will there be a small delay between incident photon and reflected photon?
Yes, we have a retarding effect, not only on reflection, but on the generic interactions between light and matter. This delay also explains the speed change of light in different media. The light itself (photon) will always be at the same speed, but within a material, the light is absorbed and emitted several times in its path, slowing its effective movement. As a consequence, we detected, at the macroscopic level, a delay in the speed of light.
Dear Zbigniew Motyka
, it seens you already answered the question. I would just add that the emission is not always isotropic, as a consequence of momentum conservation (and maybe other things) and we can calculate de scattering matrix to see which direction is most ptobably, as you alrady pointed.
Dear Zbigniew Motyka
, I mentioned momentum more in the sense of the individual process (incoming photon - interaction with electron - outcome photon), and for those processess, the scattering amplitude usually depend on the incoming and outcoming momenta. I pointed that as a attempt to figure out why a light beam have a preferential direction inside matter.A question: what do you mean by "shadow photons"?
Dear Zbigniew,
Your remark about the random reflection due to the photon absorption by the atoms going to excited states is true and good. This is known as the diffuse reflection that Marcos was in fact refering.
The reflection of light in a mirror or in a good metal is not made through the excitation by the electrons of the atoms but by the free electrons of the material, for instance in a good metal. Thus the reflection is due to a Compton scattering and there is not a delay because the photons are never absorved or emitted, they just scatter with the free electrons on the surface. This explains that the rays are parallel and all with the same angle because all the photons have the same moment and direction.
Of course-that's what the phase difference between incident and reflected wave describes! This is a standard homework problem.
I don't have the overview here - but today you have the particle-wave dualism for light (or electromagnetic radiation in general) - some phenomena may be explained by viewing light as particles (photons) (and cannot be explained as waves) and some other phenomena may be explained by viewing light as waves (and cannot be explained as particles) and some may be explained by both - but so far there is no single theory explaining it all - at least not to the satisfaction of most scientists.
Furthermore, also particles like e.g. electrons may be viewed as waves (e.g. refer to "seeing" with electrons, i.e. electron microscopes).
Thus, if someone will be able to explain all the phenomena in one complete theory - then this person or these persons may take the trip to Stockholm to receive the Nobel Prize in Physics...(!) :-)
See e.g. this Wikipedia weblink:
https://en.wikipedia.org/wiki/Wave%E2%80%93particle_duality
For diffraction see e.g. this Wikipedia weblink:
https://en.wikipedia.org/wiki/Diffraction
But even the diffuse reflection must be careful spoken about it. One good example are the fluorescence and the phosphorescence. In the case of the fluorescence the absorption of the photon by the atom or the molecule is followed by the immediate emittion, while in the case of the phosphoresce materials this emittion can be several minutes later. In the first kind of materials there is a transition between the ground state to a singlet and viceverse, while in the phosphorescence there is a transition between a single ground state to a triplet excited state which is doesn't allow a simple dipolar jump and needs a delay time.
Thus the reflection of light must be followed on what kind of material one is refering if we want to say if there is a delay or not between the ingoing radiation and outgoing one.
In the case of the phosphorescence it can be very long and of couse measurable.
Dear Jayaram,
There are very special materials which can even last the emission around a day. I show you an experimental paper where you find the measurements.
https://www.osapublishing.org/DirectPDFAccess/DCE2CAC3-C62D-F6EE-30DA717371DFF230_210172/oe-19-5-4310.pdf?da=1&id=210172&seq=0&mobile=no
Dear Zbigniew,
In your last post you do very interesting reference to time delays to graphs that I cannot see them. Finally you give an equivalent distance of 2.1 mm (70 ps) which is quite big. What do you want to say? The light cannot have any delay because its velocity is always constant and the only form to get a delay is using the electronic excitations of surface atoms or molecules. Please, could you elaborate this issue a little bit more?
In classical optics light may be reflected or absorbed. If it is absorbed first and then emitted the pattern is not a reflection as Zbigniew Motyka pointed out. It is emitted as a probability distribution that differs from a scattering profile.
A quantum effect is claimed, but the average of quantum events must agree with the classical law.
Classical optics allows calculations considering every point on a surface to act as an oscillator excited by the incoming light. Modern optics considers scattering of photons by electrons in which the sum of probability distributions is the classical reflected wave.
When a time delay occurs there is dispersion of the light and distortion of the image. Not all absorbed photons will be emitted after the same time delay.
The common decoherence of multiple quantum states with their demands for uncertainty can be modeled as light curl similarly to the Shaphiro effect in large scales. The results based on momentum conservation reveal to be a natural conclusion like in classic optics - also between two media.
That modeling is a way to try understand the difference between "absorbing-emitting" when momentum not conserved and when conserved.
Dear Zbigniew,
I'm afraid that we are not understanding each other and let me to write you in very short form.
1. The figure that you show is not for reflection of light but for comparing light reflected with transmited. The resine obviously is with a different reflection index n (although homogeneous) than the air. The question is about the delay of the reflection only which is a different question.
2. There are two light reflections: diffuse and specular. In both cases the light only can touch the atoms of the surface material.
3. The diffuse reflection takes into account the excitation electrons of the atoms on the surface of the material. Thus it takes some time the delay of the emitted light but it doesn't do it with a certain direction. I told you that this was in fact the one that Marco and you were speaking on. The exteme case is the phosporescence materials where the delay can take even days.
4. The specular reflection is the usual optic reflection of light which involves mainly the scattering of "free" electrons (for instance electrons upper the Fermi level in a metal) and therefore there is not a delay, because the "light" (photons) never can stop in this case and in fact can be considered only within the air medium.
Dear Jerry,
I understand you because in basic optics reflection is considered only as you say. But there are materials that do not absorve the radiation and in fact they re-emitte almost all the light. This reflection is called diffuse reflection and I attach you a link where they consider explicitally this kind of reflection
https://en.wikipedia.org/wiki/Reflection_(physics)
Certainly there will be a delay between absorption and reemission. Any interaction of photon with matter will take its time. That delay will be small.
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