Photons??

bogwalrus

can some one please give a nice simple analogy of what a photon is. OR a less scientific explanation.

I was reading stuff about light quanta and black-spots which got me very confused.

What i gather photons are packets of electrons that are formed due to similar high frequency vibrations giving itself mass inside a wave???

Thats what i was getting from this book but i really cant grasp it and really want to understand properties of light more=(

marco_polo

bogwalrus wrote: »

can some one please give a nice simple analogy of what a photon is. OR a less scientific explanation.

I was reading stuff about light quanta and black-spots which got me very confused.

What i gather photons are packets of electrons that are formed due to similar high frequency vibrations giving itself mass inside a wave???

Thats what i was getting from this book but i really cant grasp it and really want to understand properties of light more=(

In a nutshell, a photon is an elementry particle of physics so is not made up of anything else. It is the particle that is responsible for the electromagnetic force and carries waves such as visible light, xrays, microwaves, gamma rays etc. It has no mass which is why it can travel at the speed of light.

A quanta is a term for the smallest amount of something that can exist independently, which in the case of light happens to be a photon.

Hope that keeps you going until someone who knows what they are talking about gets here.

** Disappears before real physicists arrive **

Podge_irl

bogwalrus wrote: »

What i gather photons are packets of electrons that are formed due to similar high frequency vibrations giving itself mass inside a wave???

Photons can interact with electrons but they are a distinct particle, and are not "made up" of anything so to speak. Although the analogy isn't remotely perfect, you can think of photons as being to light what atoms are to matter.

A photon is just one quantum of the electromagnetic field, so basically the smallest partition of the EM field. You can view certain interactions of photons with matter with a very simple approach - reflection for example could be viewed as simply photons "bouncing" off a mirror or reflective surface. When you see something it is because a light/the sun has emitted photons, they've hit the object and bounced into your eye. Its very much an over-simplistic view of it, but it gets the basic idea across.
The "colour" of a photon is just related to how much energy it possesses, so when you see something green it is because there are electrons of a certain energy bouncing off it, whereas when you see something red it is because it reflects photons with different energies.

As regards electrons, photons and electrons are intimately linked, and electrons can emit photons. If an electron has too much energy, it can get rid of some by emitting a photon (this is in fact is closer to what is happening in reflection - the electrons are actually absorbing and then re-emitting photons). A photon is essentially little more then a packet of energy, so it can be absorbed by an electron - giving the electron slightly more energy, or be emitted, meaning the electron loses some energy.

IrishKnight

This might help you. It is the Simple English entry on Wiki about Photons

http://simple.wikipedia.org/wiki/Photon

marco_polo

Podge_irl wrote: »

Photons can interact with electrons but they are a distinct particle, and are not "made up" of anything so to speak. Although the analogy isn't remotely perfect, you can think of photons as being to light what atoms are to matter.

A photon is just one quantum of the electromagnetic field, so basically the smallest partition of the EM field. You can view certain interactions of photons with matter with a very simple approach - reflection for example could be viewed as simply photons "bouncing" off a mirror or reflective surface. When you see something it is because a light/the sun has emitted photons, they've hit the object and bounced into your eye. Its very much an over-simplistic view of it, but it gets the basic idea across.
The "colour" of a photon is just related to how much energy it possesses, so when you see something green it is because there are electrons of a certain energy bouncing off it, whereas when you see something red it is because it reflects photons with different energies.

As regards electrons, photons and electrons are intimately linked, and electrons can emit photons. If an electron has too much energy, it can get rid of some by emitting a photon (this is in fact is closer to what is happening in reflection - the electrons are actually absorbing and then re-emitting photons). A photon is essentially little more then a packet of energy, so it con slightly more energy, or be emittean be absorbed by an electron - giving the electrd, meaning the electron loses some energy.

Pfft! Isn't that exactly what I said?

bogwalrus

Thats savage stuff=) Just getting my head around it and trying to visualise what yer saying. Tnx


Just one Q. If photons move at the speed of light. What relationship do they have with the laws of relativity?

If something moves at the speed of light would everything be still or something like that?

I know Marco posted that photons have no mass so what part of light has mass? It is both wave and matter no?

I know i could probably go looking up the net but i find forums easier and a great way to discuss things like this=)

Podge_irl

bogwalrus wrote: »

If something moves at the speed of light would everything be still or something like that?

Photons have no concept of time. I dunno whether they'd "see" everything at once or time stand still or what - its not really a physical question. Thankfully they're not self-aware so its not really an issue!

If photons move at the speed of light. What relationship do they have with the laws of relativity?

The laws of relativity themselves are based around the fact that photons move at the speed of light relative to any intertial (i.e. non-accelerating) observer. It can be one of the more confusing concepts to get your head around. Photons are, however, affected by general relativistic events - their path deviates around a massive object for example.

I know Marco posted that photons have no mass so what part of light has mass?

They have no rest mass, photons are never actually at rest however. Again, due to relativity, you get more massive as you move faster. Photons have a momentum related to the amount of energy they carry, which again is related to the wavelength (colour) of them. Its possibly best if you think of energy and mass as (somewhat) interchangeable concepts. Your rest mass is simply the energy you have when you're not moving. As you move you obviously have more energy, which "appears" as more mass. It's more complicated then that obviously, but the concept it roughly right.

It is both wave and matter no?

Yes, but so is everything. Ultimately it is quantised, so it _is_ a particle, but light can be treated as a wave much of the time. Its all quantum mechanical, so decent analogies here are hard (and not my strong point!), but you can think of water waves - they are "waves" though there are also water molecules, so they're kinda both (this isn't the same as the photon wave-particle duality but I'm just trying to get the idea across - someone may well be able to do it better then me).

bogwalrus

what you have described above makes me even more interested in physics=)

Going to have to get a few books out so i can understand these things more in detail and maybe with some diagrams.

tnx podge_irl

kevmy

Basically on the wave/particle issue it is easier if you think of a photon as being both and neither. It is not only a wave it is a particle.

Physicists generally either think of a photon as a particle or a wave depending on which is easier/more practical for the application they are working on. It is very difficult to take in both sides of the duality and get an easy answer. Thankfully it usually works out ok if you are consistent and say it's a particle (or wave) all the way along.

ramanujan

Podge_irl wrote: »

Photons have no concept of time. I dunno whether they'd "see" everything at once or time stand still or what - its not really a physical question. Thankfully they're not self-aware so its not really an issue!

hmmm..... this got me thinking.... from our viewpoint, a photon is zero seconds "old"... as in the photons coming from your screen right now, are the same "age" as the ones coming from the stars you can see in the sky... madness.

have thought up a thought experiement to prove the above, since i'm in an imaginative mood!

Professor_Fink

kevmy wrote: »

Basically on the wave/particle issue it is easier if you think of a photon as being both and neither. It is not only a wave it is a particle.

Physicists generally either think of a photon as a particle or a wave depending on which is easier/more practical for the application they are working on. It is very difficult to take in both sides of the duality and get an easy answer. Thankfully it usually works out ok if you are consistent and say it's a particle (or wave) all the way along.

Argh! Photons are particles. They come from quantizing electromagnetic fields. The fields need not necessarily have a specific number of photons, but photons themselves are distinctly particles. These particles obey quantum mechanics, and so their amplitude can be described by a wave, but this makes it no less a particle. Many experimentalists will tell you that a photon is a click on a detector, no more no less.

Professor_Fink

bogwalrus wrote: »

Just one Q. If photons move at the speed of light. What relationship do they have with the laws of relativity?

There is no rest frame for massless particles. All massless particles (such as photons) are constrained to travel at the speed of light in all frames.

bogwalrus wrote: »

If something moves at the speed of light would everything be still or something like that?

Basically time is not defined for photons. It simply doesn't exist for them.

bogwalrus wrote: »

I know Marco posted that photons have no mass so what part of light has mass? It is both wave and matter no?

I know this has already been answered, but I thought I'd rehash it. You have almost definitely seen the equation E = m c^2. Here E is energy, c is the speed of light, m is mass in the current frame and p is momentum.

Now, in a given frame, the mass of a particle is given by the Lorentz transformation:

m = 1/(1-(v/c)^2) m_0

where m_0 is the rest mass. For a particle to exist, it must have energy (so we can't have E=0 and hence m must be non-zero). This means that for massless particles, where m_0 = 0, then for m to have a non-zero value 1/(1-(v/c)^2) must be infinite. The leads to v=c, and so all massless particles travel at the speed of light.

The energy of the particle comes from it's momentum alone.