light through medium

[caalis]

Hey.

I am new here.

Have red wikipdia and other sources from one end to other. Didn't find answere to this.

 

How exactly does light get through any medium.

I now about the reflection and refraction all the angles etc.

 

what I care for the most is...

 

weather a photon dies when encounter any phisical body or not.

 

The best answere I got in internet is that when photon encounters any atom it dies and gives extra energy to the atom, so since atom has too much energy it must spred it further (suspiciously) it creates new photon in the same direction and emits a wave in the same frequency as received photon-wave.

 

this has a logical mistake

1. Why does atom spreads out energy in the same direction and in the same frequency as it recieves it?

2. This explanation proposes that everything on this planet is transparent. It isn't!!!

 

Can someone help me with this. any link to an internet site, or explain by yourself.

 

Thanks in advance...

[Klaynos]

The whole thing is quantum mechanics.

 

Everything discussed has a wavelength dependence.

 

There are 3 processes which need to be considered, absorption, reflection and transmission. For any photon it may be absorbed, or may not be, if it is it increases the energy of one electron, depending on the material it's energy may then be dissapated in phonons and other non-light ways, or it may give out a photon, either in the same direction or another direction, this is either a transmission or reflectance event, depending on the angle.

 

The angle depends on quantum mechanics very strongly and it's difficult to explain without knowning how much QM you are familiar with?

 

I'd suggest reading:

 

http://en.wikipedia.org/wiki/Fermi's_Golden_Rule

and

http://en.wikipedia.org/wiki/Fresnel_equations which is more wave mechanics bassed than quantum mechanics what's really going on.

[5614]

The best answere I got in internet is that when photon encounters any atom it dies and gives extra energy to the atom, so since atom has too much energy it must spred it further (suspiciously) it creates new photon in the same direction and emits a wave in the same frequency as received photon-wave.

 

this has a logical mistake

1. Why does atom spreads out energy in the same direction and in the same frequency as it recieves it?

2. This explanation proposes that everything on this planet is transparent. It isn't!!!

Well your explanation is, in a very simplistic way, about right.

 

A photon hits an atom and knocks an electron up to a higher energy level. This "knock up" is exactly equal to the energy of the photon. The photon no longer exists, it's energy has gone into the atom (more specifically the electron). But the atom is no longer stable, because the electron is in a high energy level, so the electron falls back down to where it was before. This realises energy exactly equal to that "fall down", which is equal to the "knock up", which is equal to the photon's energy.

 

1) I don't have anything to add to what Klaynos answered above

 

2) no, because different things will absorb/emit photons in different ways. This comes down to the structure of the material.

 

So for example my blue wall reflects most blue light (i.e. light hits the wall, then "bounces" off it), other colours are all absorbed. Whereas light that hits the glass in my window will be absorbed by the glass, but then emitted in the same direction it was travelling. So glass is transparent, whereas a wall isn't.

[swansont]

Well your explanation is, in a very simplistic way, about right.

 

A photon hits an atom and knocks an electron up to a higher energy level. This "knock up" is exactly equal to the energy of the photon. The photon no longer exists, it's energy has gone into the atom (more specifically the electron). But the atom is no longer stable, because the electron is in a high energy level, so the electron falls back down to where it was before. This realises energy exactly equal to that "fall down", which is equal to the "knock up", which is equal to the photon's energy.

 

However, there is a big difference in that there are two possibilities for the absorption. It can be to a virtual state, in which case the photon must be re-emitted with the same energy and polarization and in the same direction; this takes time, so the propagation speed is smaller, and is what happens to the light that makes it through a material. If the absorption is by a real state, the emitted photon can go in other directions, and the light is not transmitted.

[granpa]

a light wave like all waves can be seen as moving by means of 'wave generators'. these generators are at every point of its wave front. each produces a wave that spreads out in all directions but they cancel in all directions except forward so that the resulting wave exactly reconstructs the original wave one step ahead of where it started. this is useful when visualizing how it will behave when diffracting around a corner.

 

in a conductor (metal) the electrons are free to move and therefore no electric field can penetrate it for more than an infinitesimal time because the electrons move and generate a field that exactly cancels out the penetrating field. the result is that light is reflected off its surface. in a dielectric the electrons are bound and therefore not free to move beyond certain limits. surprisingly a dielectric behaves exactly the same way a conductor does just to a lesser degree. some of the light will be reflected off its surface. the rest will continue to travel on through the material at a reduced speed. be aware though that there is a group velocity and a phase velocity.