Description of a photon

[John0117]

Rather than sidetrack my other post I will ask the question here.

I have been unable to find any useful description of the word photon other than discrete bundle or wave-packet neither of which are very helpful. The descriptions available seem to describe a particle like structure rather than a wave like structure.

So how are photons and waves connected, what relationship does the energy of a photon have with wavelength. If light travels as photons why bother to describe it moving as Electro-magnetic waves.

 

[Delta1212]

Particles and waves are not an either/or thing on the quantum level. A photon is both a particle and a wave. (Or, more accurately, has properties of both a particle and a wave).

[swansont]

E = hv =hc/lambda (v = frequency, lambda=wavelength)

 

Light diffracts and interferes, which are decidedly wave-like properties. But the energy is quantized and the interaction can be localized to something much smaller than a wavelength, which is particle-like.

[Orodruin]

To add a bit on the why bother issue: when you have a lot of photons the effects on the macroscopic level are well described by classical field theory of electromagnetic waves. Quantum effects become averaged out. At that level it is simpler to treat the EM wave and it still gives a good description - much like newtonian gravity is fine for everyday life without the need to go to GR.

[John0117]

I am trying to picture a photon. E=hv and wave length is relatively easy. A wave being wavey and energy linked to its length. Obviously a wave means an EM type wave with electric and magnetic fields oscillating at right angles.

So how would a photon appear, as a square perhaps for want of a better word.

In the dual slit experiment a single photon is used how would that differ from a single wave as it still produces an interference pattern as a wave would.

Sorry to be a pain but I am just trying to get it right. Better to ask silly questions than get the wrong idea.

[Orodruin]

A photon in itself is a quantum field theoretical object which makes it difficult to picture in terms we are familiar with. Just as a quantum mechanical particle does not have a well defined position or is a classical wave, a photon is also struggling with this. A better (for many purposes sufficient, but still not fully accurate) description of a photon would be in terms of a wave packet for its wave function.

[Janus]

I am trying to picture a photon. E=hv and wave length is relatively easy. A wave being wavey and energy linked to its length. Obviously a wave means an EM type wave with electric and magnetic fields oscillating at right angles.

So how would a photon appear, as a square perhaps for want of a better word.

In the dual slit experiment a single photon is used how would that differ from a single wave as it still produces an interference pattern as a wave would.

Sorry to be a pain but I am just trying to get it right. Better to ask silly questions than get the wrong idea.

A single photon won't produce an interference pattern. It will produce a single dot on the screen. However, if you continue to send photons through, one at time, each one will create an individual dot on the screen, but as the dots pile up, they will slowly form an interference pattern.

 

A classical wave will produce an full interference pattern no matter how much you reduce the light, it will just darken as more light hits the screen.

A classical particle will produce individual dots on the screen, but as more dots are added, no interference pattern appears.

 

Light will produce individual dots that fill in to form an interference pattern, thus its dual nature.

[John Cuthber]

Here's a picture of an animal (sadly extinct)

http://en.wikipedia.org/wiki/File:Quagga_in_enclosure.jpg

 

Sometimes it doesn't look like a zebra- for example from behind

Sometimes it doesn't look like a horse - for example- from the front

 

Because of these two properties we know that it's not a horse and not a zebra (actually, it's a quagga)

 

Sometimes photons don't look like waves- for example in the photoelectric effect.
Sometimes they don't look like particles- for example in diffraction experiments.

 

Because of these two properties we know that it's not a wave and not a particle (actually, it's a photon).

 

If you try to understand quaggas by pretending they are horses or zebras, you are not going to get very far.

a quagga is not " a mixture of a horse and a zebra"

If you try to understand photons by pretending they are waves or particles you are also not going to get very far.

a photon is not " a mixture of a wave and a particle"

 

A photon doesn't have a "dual nature" any more than a quagga does.
It has one nature - it's a photon.

 

.Any attempt to say "they are like a so and so" will be an analogy rather than the real thing.

They are different from anything you know about.

Edited May 6, 2014 by John Cuthber

[John0117]

Some good information.

The quagga actually fits in with a thought I had this morning and that is

Light looks like a wave from the side and a photon end on.

Imagine the difference between an arrow approaching you sideways and end on.

The shaft producing the interference pattern and the head producing the dots (from post 7)

The shaft representing the energy carried and the head the momentum.

Would that possibly be a reasonably accurate picture?

[Mordred]

no not really an accurate way to describe it lol. A particle in QM has both pointlike properties and a wave function. If you took a laser beam and fired it at a slit it would have a dispersion pattern that can only be generated by a frequency wave. If the particle didn't have a wave function it would consistently hit the same point. Particles are described by its spin, momentum and energy. Energy has a mass equivelence. However detection of particles is done by shooting electrons or other electromagnetic medium at a particular medium. The electons then have a correlating dispersion pattern due to the interaction with that particle. From this dispersion pattern we can infer the properties of the particle itself, as well as the structure of the atom for example. The pointlike properties is a descriptive of the region of highest energy density. Often referred to as a packet. We cannot describe how much volume a particle has, we simply have no means of directly measuring volume of a particle. They are too small and move too fast. To work around this problem QM developed its metrics upon the information we can successfully collect. Spin, momentum, and energy. Frequency is a property of its momentum. Spin is due to the configuration of quarks and gluons that make up the particle. Google eightfold way. http://en.wikipedia.org/wiki/Eightfold_Way_%28physics%29

 

A good textbook covering particle physics is Inroductory to Elementary particles by David Griffith, he also has a good introductory book on quantum physics.

http://www.amazon.ca/Introduction-Elementary-Particles-David-Griffiths/dp/3527406018, you can see the other books on the same page

[MigL]

Thanks John and Mordred, a quantum particle has a definite definition , but it's not a classical particle or a wave.

A lot of the wave/particle duality results depend on the experiment set-up.

If you set-up detects wavelike behaviour you will 'see' waves, and if it detcts particle like behaviour, you will 'see' particles.

Just like looking at pictures of your extinct animal from different directions John.

[Enthalpy]

If light travels as photons why bother to describe it moving as Electro-magnetic waves.

 

 

Apparently you'd like "particle" to mean "well-located point". This is not what quantum mechanics has kept from the idea of particle.

 

Photons, light, travel exactly as a wave, decribed by the electromagnetism equations, which must hence be kept.

 

Light particles, or photon, serve to account that light appears and disappears in integer multiples of an energy h*F. There isn't much more that that behind the idea of photon. Even the absorption of a photon does not need to be local; when a 10m long antenna absorbs a photon, the position isn't more accurate than 10m. Or when a semiconductor photodetector absorbs a light photon, it uses to happen over many thousand atoms.