Photons

[AtomicMX]

I heard that a photon cannot be accelerated, but.. then what happens to the energy applied... does it turns back...or what...

 

It is supposed that speed of light is a constant specified by photon.... but then again.... when the photos has its particle moment could it be acelerated... or is this the changes between particle and wave... an aceleration and desaceleration...

 

And .... is the speed only messurable for matter?

[timo]

- photons all move with the same speed.

- energy is inversely proportional to the wavelength of the photon. Incresing it´s energy reduces it´s wavelenght.

[AtomicMX]

energy thinking in hrtz?

and well.. then... how can the wavelength vary if they move at the same speed and go through the same space at same time.... (how could some paralels photons) have diferent wavelenghtd moving at same speed

[Mr Intron]

frequency · wavelength = c

Where speed of light is constant (although depending on the medium). Which means a higher frequency gives rise to a shorter wavelength.

[JaKiri]

energy thinking in hrtz?

and well.. then... how can the wavelength vary if they move at the same speed and go through the same space at same time.... (how could some paralels photons) have diferent wavelenghtd moving at same speed

 

E = hf, c = f lambda

[AtomicMX]

well i knew that for electromagnetism but the speed of light its a constant... how is it posible to get a formula out of it...

 

Where speed of light is constant (although depending on the medium).

 

.... can you explain that please

[timo]

Wave equation of a free particle is given by:

[math] \Psi (\vec x, t) = C \cdot \exp \left( \frac{i}{\hbar} (\vec p \vec x - Et) \right) = C \cdot \exp \left( i (\vec k \vec x - \omega t ) \right) [/math]

with k being the wave-vector and omega being the frequency of the wave (times 2 pi).

 

The relativistic relation between momentum and energy of any particle is:

[math] E^2 = p^2 c^2 + m^2 c^4 [/math]

 

Since for a photon m=0 it follows that:

[math] E = |\vec p|\cdot c [/math]

 

So for the speed of the wave it follows that regardless of E and p:

[math] v = \frac{\omega}{|\vec k|} = \frac{E}{|\vec p |} = c [/math]

 

 

Sorry that I can´t really understand your problem but I hope above helps you.

 

EDIT: Corrected sign-error in 2nd equation

[Dave]

Ack, it's like my quantum phenomena module all over again.

 

If you didn't understand that, just take it from us that it's true.

[YT2095]

a good way to think of energy applied being proportional to the photon frequency is to think of a peice of metal over a gas flame, at 1`st it`ll go a black color and give off "black body radiation" infrared, then after a while when it`s had more heat energy it`ll glow red (visible spectrum and a shorter wavelength) then it`ll turn orange then yellow then probaly melt, but with more energy added the frequency gets higher and the wavelength shorter. the same happens with a lightbulb it goes from dull red to white as you increase the Voltage (energy).

 

hope that helps a little?

[5614]

on an atomic scale.....

 

how are atoms made?

&

how do they react with matter?

 

e.g. what happens when the photon hits the item?

 

ON AN ATOMIC SCALE, like what happens to the atoms/photons

[YT2095]

I can answer the middle bit, Atoms ARE matter, they can then join to form elements and/or chemical compounds (molecules).

 

when light (a photon) hits some of them an electrical charge can be given off, some chemical compounds exploit this phenomenon better than others and are thus used in solar cells and pannels.

 

I appreciate this is only a simplistic answer, but I`m no Physicist and can only give you part of the story from a chemists point of view.

[5614]

when light (a photon) hits some of them an electrical charge can be given off' date=' some chemical compounds exploit this phenomenon better than others and are thus used in solar cells and pannels. [/quote']

 

how can an electrical charge be given off, for that the electrons have to move, and that is what i was getting at,

do the electrons move, if so: who, how, what, when, why, where etc etc?!?!

[timo]

@5614:

 

- how are atoms made (on an atomic scale)?

Atoms consist of a positively charged nucleus (whose structure is on a subatomic scale and thus irrelevant here) and electrons bound to the core due to electromagnetic interaction. Due to boundary conditions (electrons with a low energy can´t get away from the nucleus) the electrons can only be in certain states with definite energy. There is only a valid state for certain "quantitized" energies.

On an atomic scale it seems best to treat the electrons with QM and the atom as a whole as a classical particle.

 

- what happens when the photon hits the item?

As someone said in your thread about that topic a lot of things can happen. That´s most probably why no1 gave you a definite answer. In above picture imagine the photon as a packet of energy and momentum. Energy and momentum can be transferred to the atom as a whole which causes it to move. Energy (and no momentum) can be transferred to an electron to transfer it in an other bound state with higher energy. Also, an electron can be given enough energy to become free of the nucleus and move away (it´s also given momentum, then). All mixes of those effects that conserve energy and momentum can happen.

At higher energies photons can also interact with the nucleus resulting in the additional option to put the nucleus in another state (energy differences of the nucleus-levels are bigger than those of electron-levels). Also the usually not very important, yet interesting, effect of pairbuilding (is that the correct english expression?) can happen: That´s when the photon "decays" into an electron and a positron (anti-electron).

[YT2095]

how can an electrical charge be given off' date=' for that the electrons have to move,

 

that`s exactly what does happen

 

electrons are displaced and picked up on a conductive layer

[5614]

thanks atheist, can u explain what yt2095's saying, i dont get it!?!?!? soz yt!

 

what are the electrons displaced with, what takes their place???

what conductive layer, and surely the photon [energy] cant displace matter [electrons]

[timo]

My answer refers to a single isolated atom. YT refered to solid bodies consisting of many atoms/molecules. Explaining solid state physics would lead a bit too far, I think, but:

- sooner or later an electron will take the place that became free (usually by emmiting a photon in this process).

- "conductive layer" is a certain subset of states in which electrons are not bound to an atom and only bound to the body as a whole.

- Energy can DISplace charges. It cannot REplace them.

 

EDIT: Even though I don´t think I´m able to explain solid state physics correctly and in an easy way I should add that several effects (like solar cells) of photon<->matter interaction can only be discribed within the scope of solid state physics. The princliple "particles are sent to a different quantum mechanical state" (usually by absorbing part of the photon´s energy and momentum, but also by releasing other photons as in lasers) remains the same.

[5614]

cool, so how are photons produced, i think it may be something to do with electrons switiching shells, but i might be wrong

[ed84c]

I think what atomicMx meant was that if a photon travels in a path like a wave then a shorter wavelength would mean that it would have to travel farther providing the amplitude was the same. And then if the overall 'as the crow flies' measurement of distance it would require the photon with the higher frequency to e traveling faster than the one of lower frequency. Rather like 2 cars travelling from a to b, if one is on the motorway and one in twisty back roads, if they are at the same speed, then the car on the motorway would get there sooner as it has had to travel less real distance, although they have had to travel the same a to b.

 

But im not sure photons work quite like that.....

[JaKiri]

They don't. It's point to point speed.

[timo]

cool, so how are photons produced, i think it may be something to do with electrons switiching shells, but i might be wrong

 

No that´s correct. Replace "shells" by "quantum states" and you almost got what I was saying. Electrons can switch to an energetically higher state (conservation of momentum let aside for now) by absorbing a photon and switch to a lower state by releasing a photon.

Since the energy differences (the ammount that is absorbed from a photon or released as a photon) between the electron's possible quantum states are different for different atoms you can identify atoms by the wavelength of the photons they emmit (-> spectral lines).

[swansont]

cool, so how are photons produced, i think it may be something to do with electrons switiching shells, but i might be wrong

 

That's one way, as has been explained. Accelerating charges is another (that's how e.g. radio waves are produced with an antenna, or x-rays by slamming electrons into a metal plate and rapidly stopping them). You also get photons by particle/antiparticle annihilation (mass converted to energy)