The speed of light in a medium

[Jacques]

Hi

I readed in many threads in this forum that the speed of light is constant even in a meium like water or glass. That the apparent slowing down is cause by the photons being absorbed by the atoms of the medium and after a moment being re-emitted. The speed of the photon between atom is always c.

My question is Why doesn't the light is scattered, dispersed ? How does the atom remember from which direction the photon came ?

Thanks

For your answers

[Xyph]

The atom doesn't remember anything. If a photon hits a nucleus it will be deflected or absorbed, and if this happens a lot I assume the light will be scattered and dispersed. Just remember that atoms are mostly empty space though, so there's plenty of room for the photons to move in without hitting anything.

[5614]

And then how does a molecule know always to reflect so that i=r ???

 

I've always wondered this and the original question.

[CPL.Luke]

why does the E field remember to get weaker at the rate of 1/r^2 ?

[Jacques]

so there's plenty of room for the photons to move in without hitting anything

I know.

How do you explain the change in speed of the light in a medium ?

I was refering to some explainations given to that in this forum.

[5614]

How do you explain the change in speed of the light in a medium ?

Because photons are continuously absorbed and reemitted, this processes take time, we're still talking tiny amounts of time, but when referring to light a small time makes a noteable difference.

[Jacques]

Because photons are continuously absorbed and reemitted,

Then why doesn't the photons scattered ? If a photon is absorbed by an atom it can be reemitted in any direction, not necessarily in the same direction.

[J.C.MacSwell]

I always pictured light staying at "c" but having further to travel (around molecules) in different mediums, but also have been puzzled by this "absorbtion/readmission".

 

Do photons have a much greater likelyhood of being readmitted in the same direction they were going (prior to absorbtion) in transparent materials?

[swansont]

Then why doesn't the photons scattered ? If a photon is absorbed by an atom it can be reemitted in any direction, not necessarily in the same direction.

 

AFAIK it's because the absorption is in virtual states, and conservation of momentum requires them to be reemitted in the same direction. Scattering happens when the interaction is with a real state.

[5614]

What do you mean by a virtual state and a real state?

[x__heavenly__x]

Well, This absorbtion and reemision is just an analogy, in reality photons , which r just vibrational energy(not at all particles, like in the real sence) just hit go through a medium and get deflected, just as they enter a medium, u know refraction happens at the surface. Ok as it hits the surface, they get an effect of being thrown out of their original path. and in the medium continue in that deflected path, when they exit, they get deflected in the same way but reverse....so original path again....

 

But there was something which always bothered me...light will get diflected only to the plane its traveling....like a line of laser in a 2d plane, traveling at x+y will get diflected in the x-y plane not the z plane, even though in 3d world...i donno wat happens.

[swansont]

What do you mean by a virtual state and a real state?

 

A real state is one of the quantized energy states you would get if you could solve the Schroedinger equation for that atom in question, e.g. the 10.2 eV for the first excited state in Hydrogen.

 

Monochromatic light, far from any absorption line, still sees an index n>1 when passing through a vapor. The photons are not absorbed in a transition that puts the electron in an allowed state, so this can only happen for a short time. Consequently, the photon can't transfer momentum to the atom and has to be re-emitted in the same direction. (Heisenberg Uncertainty Principle)

 

Classically it's just the interactions with the fields in place that change the permittivity and permeability, slowing the wave down.

[GeminiinimeG]

I have no idea what I am typing but here it goes, the gravity within the atoms affects the light? or maybe the light is being slowed down by all the atoms in its way, how does this work???? Maybe the photons bounce around within the water and seems that the ones that make it thought to the other side will be dimmer and weill seem to be slowed but its just many of them were absorved or they bounced off somewere else. am i correct

[5614]

I have no idea what I am typing

That is correct.

 

am i correct

No.

 

how does this work????

Try reading the thread, even the original poster stated it!

 

btw, gravity has not been observed on a subatomic level, it is too weak as a force.

[x__heavenly__x]

A have to agree with 5614.

 

Am i evil?