I don't understand exactly how the propagation of light works

[lordoftheselands]

Hello, people say that light has properties of particles and waves. Ok, but how exactly?

We have the principle of rectilinear propagation of light as a law on Physics. Light would be a particle that propagates in a direct line pointed to all directions with origin on the source of light.

The problem is that this is impossible. Light can't be emitted in all direction in a rectilinear way. In case you stand 1 meter far away from the light source you could trace X directions. But in case you stay away 2 meters from the light source you would trace X + Y directions. And in case you stay away 3 meters away from the light source you could trace X + Y + Z directions. How can a photon be emitted in all directions rectilinearly if the amount of directions increases according to the variance of the distance? I'm being very nice saying that there are X directions when you stay 1 meter away from the light source, there are actually infinite directions even from that distance.

So light can't be a particle. It's a wave. Because waves go to all directions and they actually increase in size. A single wave gets bigger and bigger as long as it goes away from a certain origin...

It's impossible that infinite photons are emitted from the source of light in all directions since there is no energy for this.

the wave gets bigger and bigger meaning that there would be more and more photons originating from nothing as long as the wave keeps going away from its point of origin

how many photons are emitted in a single wave originated from a light source? you can't say it because the wave gets bigger and bigger so the number of photons increases

Help me guys, someone could point out how exactly does light propagate?

 

[256bits]

So light can't be a particle. It's a wave.

how many photons are emitted in a single wave originated from a light source?

Do not mix up quantum description of light with the Maxwell wave description of light.

I'm being very nice saying that there are X directions when you stay 1 meter away from the light source, there are actually infinite directions even from that distance.

So the previous sentences are not necessary. There are infinite angular directions in a plane no matter what distance from a central point, and infinite number of planes that have the central point as one of its point members.

 

[Drakkith]

Light behaves almost exclusively as a wave when traveling. Its 'particle properties' only really show up when it interacts with something (like a ccd detector). In reality, we can't even give the photon a position when it is traveling, so it makes even less sense to think of it as a particle. I'd simply stick to treating light as a wave unless you're doing ray optics or something where you simplify things into neat lines and ignore wave effects like diffraction and interference.

how many photons are emitted in a single wave originated from a light source? you can't say it because the wave gets bigger and bigger so the number of photons increases

Strictly speaking, this isn't true. There's some wiggle-room because of the uncertainty principle, but if your emitter emits one or more EM waves with a 500 nm wavelength, using 1 joule of energy, then you know you have about 2.5x10^18 photons. As the waves spread out, they deposit less and less energy (fewer photons) into any given area. With enough distance the wave has so little energy per area that a detector would probably not detect a photon at all as the wave passes by.

This is, in fact, what happens when we do astrophotography of distant targets. The energy density in the emitted EM waves is so low that we have to have hours and hours of exposure time just to get a few dozen photons from the target.

 

[256bits]

I knew somebody would come in and have a much better explanation and discussion than I.

 

[sophiecentaur]

In reality, we can't even give the photon a position when it is traveling, so it makes even less sense to think of it as a particle.

This caveat just doesn't seem to be read (or at least remembered) by the uninitiated. People hang on to the mental picture of a little bullet whatever they are told. It's such a shame that people who have sorted all this out correctly and who used photons in their daily studies of Physics don't seem to realise the damascene conversion they must have gone through in their learning process.

Even the poster boy of modern Physics invented the Feynman Diagram without making it clear that it's just symbolic and that it's not a 'picture' of what's happening.

That squiggly line says nothing at all about the nature of the EM energy as it goes between the (actual) particles. I guess it must be that tangible particles are more approachable than airy fairy waves.

 

[lordoftheselands]

I still don't understand how light can be emitted in infinite directions.... seems impossible... at least if it's a particle... if it was a wave that would be better explained....

just think about observers standing billions of light years away in millions of directions... there would be photons emitted rectilinearly in all these directions... sounds like magic for me

 

[Dale]

if it was a wave that would be better explained.

Did you not see post #3?

Just stick with the wave description until you get to an actual class on quantum electrodynamics. Until then your mental model of a photon is going to be much more wrong than just treating it as a wave.

 

[russ_watters]

This just looks like simple, bad geometry to me. The number of directions is always infinite regardless of distance. The number of photons is very large, but finite. So what? Where is the problem?

[Edit: post #3 is more detailed...]

 

[lordoftheselands]

This just looks like simple, bad geometry to me. The number of directions is always infinite regardless of distance. The number of photons is very large, but finite. So what? Where is the problem?

[Edit: post #3 is more detailed...]

the problem is that photons are emitted in all these infinite directions...seems impossible, since the number of photons are finite like you said

regardless of where you are, when you look at the source of light you would see it... seems that something isn't well explained there

 

[Ibix]

the problem is that photons are emitted in all these infinite directions

Are they? How can you tell? You would need an infinite number of detectors and to confirm that all of them triggered. You seem to be aware that they would not all trigger even if you could build an infinite number of them, so you seem to have found the flaw in your thinking.

 

[russ_watters]

the problem is that photons are emitted in all these infinite directions...seems impossible, since the number of photons are finite like you said

That's your misunderstanding: they are not emitted in all directions.

regardless of where you are, when you look at the source of light you would see it...

That isn't what happens. It is possible to not see a light source because its photons miss you.

 

[Vanadium 50]

Even the poster boy of modern Physics invented the Feynman Diagram without making it clear that it's just symbolic and that it's not a 'picture' of what's happening.

Huh?

I don't think you mean Ernest Stückleberg. If you meant Richard Feynman, the paper where he introduces them, The Theory of Positrons, PR76 6 p.149 (1949) is very clear on the issue. The fact that there are an order of magnitude more equations than diagrams shows where the paper's emphasis lies.

 

[Drakkith]

the problem is that photons are emitted in all these infinite directions...

That's simply not true. Consider an emitter that emits an EM wave with enough energy for only 10 photons. This wave is completely surrounded by 100 photomultiplier detectors with 100% efficiency at detecting photons, each placed 1 meter away from the emitter. At best, only 10 detectors will register a detection. There simply isn't any more photons to detect. Of course, we could have 6 detectors register a single photon and 2 register two photons. Or 5 detectors might register two photons each. Or, in a possible but very improbable situation, a single detector registers all ten photons.

The point is that the photons are detected in a statistical manner, not in a one-photon-per-geometrical-line manner. This 'photon noise' or 'shot noise' (the random chance that a photon is detected) is of extreme importance in imaging, especially low-light imaging, since it is the most fundamental source of noise. By that I mean that all other sources of noise can be mitigated and/or improved, but not shot noise. Thermal noise can be improved by cooling the detector or improving its design. Background noise can be improved by moving to low-background sites. Noise from electrical interference can be improved by shielding or better circuit design.

But shot noise is forever.

 

[sophiecentaur]

seems impossible... at least if it's a particle.

You have to specify exactly what is meant by 'particle'. Certainly not a tiny bullet.

If you meant Richard Feynman, the paper where he introduces them, The Theory of Positrons, PR76 6 p.149 (1949) is very clear on the issue.

It's obviously not clear enough but that's no surprise. The Feynman diagram is a very elegant way of showing the logistics of interactions but it should not be interpreted as a photograph of what happens. My earlier point was that this (very attractive) interpretation is just what people make, which is where the problem starts. With the 'Feynman badge' it is even more likely to be held onto with grim determination. It's a sort of 'ad hominem' reaction; "My interpretation is what Feynman said so it must be true".

 

[dsaun777]

Did you not see post #3?

Just stick with the wave description until you get to an actual class on quantum electrodynamics. Until then your mental model of a photon is going to be much more wrong than just treating it as a wave.

It's a wave that transfers energy discretely. That discreteness is the "particle." Is that accurate?

 

[sophiecentaur]

It's a wave that transfers energy discretely. That discreteness is the "particle." Is that accurate?

Yes. That’s a good way to look at it. The particle behaviour (momentum etc) kicks in during the interaction with source or ‘receiver’ which are specific locations.

 

[Nugatory]

the problem is that photons are emitted in all these infinite directions.

A better way of thinking about is that there are no photons unless and until the light interacts with something.
We have electromagnetic waves radiating out from the source, no photons, no particles. Naturally when this radiation interacts with something (photographic film, a CCD detector, the retina of your eye, ….) it transfers some of its energy to that something, like waves pounding on a seashore. However, experiments have shown that, unlike ocean waves, when electromagnetic waves interact with matter the transferred energy is always deposited in tiny discrete amounts at single points - and whenever that happens we say “a photon’s worth of energy landed there”. But unless and until that interaction happens all we have is the electromagnetic wave.

 

[Mister T]

Light can't be emitted in all direction in a rectilinear way. In case you stand 1 meter far away from the light source you could trace X directions. But in case you stay away 2 meters from the light source you would trace X + Y directions. And in case you stay away 3 meters away from the light source you could trace X + Y + Z directions. How can a photon be emitted in all directions rectilinearly if the amount of directions increases according to the variance of the distance?

Photons do not behave like little bullets. But let's substitute bullets for photons. If you were located 1 meter away from a spherically symmetric distribution of bullets coming from a meter away, and then you moved to 2 meters away you would simply have fewer bullets per unit area.