Understanding Light: The Duality of Particles and Waves

[Chipper]

Ok so my grade 12 physics class is focused around what light is. I am sure this has been asked a lot but i can't figure it out. Everywhere i turn i get an uncertain answers. So which is it? is it both? Is there a simple explanation i can use to explain to my colleuges?

 

[Pengwuino]

Yes, there's a very simple explanation: It's both.

It exhibits wave-like properties but it also exhibits particle-like properties.

 

[Chipper]

see I've been told that but i can't seem to grasp it. WHen i see a wave i see a continuous thing but a particle is more like a dot.. so then is it safe to say that light is parts of waves? Like your local atm? You can only take out values of 20$. You can have 47.50$ but you can't take that out. Is that a reasonable analogy?

 

[Pengwuino]

No that's not very reasonable. I can't really explain it to you... or well, most people can't explain it very well. I think the only reason I can even begin to grasp it is when I look at the implications of how light works.

 

[Geoff St. Germaine]

see I've been told that but i can't seem to grasp it. WHen i see a wave i see a continuous thing but a particle is more like a dot.. so then is it safe to say that light is parts of waves? Like your local atm? You can only take out values of 20$. You can have 47.50$ but you can't take that out. Is that a reasonable analogy?

The best way to think about it I find is that it isn't a particle or a wave. It simply appears to be more wave-like or particle-like depending on the experiment. The same thing is true of particles also. There are experiments where electrons behave like waves. My opinion is that the wave-like and particle-like are simply ways of relating how we observe things on quantum levels with what we are used to seeing on the macroscopic levels.

 

[G01]

Light is both wave and particle. You can never see light being both at the same time though. Like if a photovoltaic cell absords light to make electricity it absorbs fixed packets of light, hence particles of light. But on the other hand when you see different colors of light, you are seeing different frequencies of light, hence you are seeing light as a wave.

 

[Reugar]

Basically what has been said here is correct. Particle/wave duality, in your physics class you will most likely do the "dual slit" experiment, which should help you to understand a little better. When I was first learning about light, I read and listened to some of the Feynman lectures on it, he does a very good job of explaining it, so I suggest you give that a try. Cheers.

 

[pervect]

Ok so my grade 12 physics class is focused around what light is. I am sure this has been asked a lot but i can't figure it out. Everywhere i turn i get an uncertain answers. So which is it? is it both? Is there a simple explanation i can use to explain to my colleuges?

Light is a strange beast. Think of it as being whatever it is. In the high energy limit, it acts like particles. In the low energy, many particle limit it acts like a wave. In between, one must use the mathematics of quantum mechanics to describe it.

As far as simple explanations go, you might try Feynman's book "QED". Don't expect really simple answers though - Feynman, who one the Nobel prize for his work in quantum mechanics, is on record as saying that "Nobody really understands quantum mechanics". This should not be interpreted as saying that we don't have an adequate mathematical formalism to describe how light behaves, it should be interpreted as meaning that the mathematical description is not reducible to an intuitive one in terms of familiar, classical objects.

(To put it another way - classical objects are an idealization of a more complex underlying phenomenon. This is my $.02, not Feynman's theme.)

"QED" is probably one of the more approachable books which really discusses the strangeness of QM, however.

 

[nevetsnosaj]

wave and particle combined

We have to accept the fact that light is both wave and particle. The fact that light is wave is that we can see color spectrum. The wave of light will be separated into 7 diffrent colors. ROYGBIV.

In the other hand, the fact that light is a particle can be seen from photosynthetis. The leaves absorb the light to convert the H20 and other mineral for their growth.

 

[KingNothing]

I had a very hard time grasping this at first two - what I kept doing was I'd try to imagine a wave made of particles, or something like that. It doesn't make a lot of logical sense, no. But science is based on what we can measure, and we can measure it as a wave and a particle.

 

[Curious6]

Light can also be described as a particle which exhibits both particle-like and wave-like properties.

 

[MonstersFromTheId]

The real problem here is this...

Light doesn't consist of either "particles" or "waves".
So what does it consist of?
NOBODY REALLY KNOWS, and that's the real problem with trying to explain "what light is made of", and the reason you'll hear people talking all the time about "wave/particle duality".

You see the term "wave/particle duality" is really just a fancy way of saying that sometimes it's useful to THINK of light as being made up of particles (even though it isn't, not really), but since it acts, JUST LIKE it's made out of particles, pretending it's made of particles is a really good way to predict its behavior, but at the same time, it's often useful to THINK of light as being made up of waves (even though it isn't, not really), but since it acts, JUST LIKE it's made up of waves, pretending it's made up of waves is also a really good way to predict its behavior.

Now the temptation is to just throw up your hands and think "Well all this is just baloney!" But it really ISN'T "just baloney", and it really IS worth learning about, because even though nobody knows exactly what "light is really made of", we DO know one HECK of a lot about how whatever light is "really made of" will behave.
For example:
Pretending that light is made up of waves, (even though we know it's not), is still useful because whatever light really IS made of ALWAYS behaves EXACTLY like it IS made up of waves. So by pretending for a moment that light is made up of waves is really really REALLY useful. It's what allows us to design things like lenses.
At the same time...
Pretending that light is made up of particles, (even though we know it's not), is still useful because whatever light really IS made of ALWAYS behaves EXACTLY like it IS made up of particles. So by pretending for a moment that light is made up of particles is really really REALLY useful. It's what allows us to design things like solar cells and lasers.

So the answer to your question - "Is light made of particles or waves?" is - it isn't made of either waves or particles, it's made of something that behaves like waves and behaves like particles all at the same time.

 

[TheAntiRelative]

So, is it wrong to see it like a traffic simulation?

IE: The cars are the photons and the waves of cars as they stop and start down the line are the wave portion so traffic is both wave and particulate?

 

[Toptomcat]

Yes, that's wrong- because a single car/photon exhibits wave-like behavior.

 

[MonstersFromTheId]

Yeah, that'd be a bad analogy.

I.e. "The cars are the photons and the waves of cars as they stop and start down the line are the wave portion so traffic is both wave and particulate."

And here's why - the particles (the cars) always always ALWAYS move at *exactly* the same speed (c = 186,000 miles / second). That never changes.
Even when light "slows down" as it moves through air or glass, the particles (photons) STILL move at exactly the same speed (c).

Now I know that sounds crazy, but it actually makes sense if you look at what happens when light moves through vacuum, as opposed to what happens when it moves through something like, say, glass.

When light moves through vacuum the photons have nothing at all to run into. So they just zip along, pefectly happy, at the ONLY speed they EVER move at, which is "C", the "speed of light in a vacuum", which is 186,000 miles /second.

But when light moves through glass the photons have PLENTY of things to run into. They have gazzillions of glass molecules to run into. And each and every time the photons run smack into one of the atoms of a glass molecule they have to take time to go through an energy exchange process.

1) The photon hits one of the atoms in one of the glass molecules.
2) The photon turns into energy, pure "kinetic" energy, as in the photon TOTALLY ceases to exist. For a split second there is no photon.
3) That energy boosts the electrons of the atom it hit into a higher orbit.
Picture a tether ball (an electron) swinging around a pole on a string (the "pole" being the nucleus of the atom). When the photon hits the electron it's like smacking the tether ball as it goes by to make it go around the pole even faster.
4) But the electrons won't stay in that higher orbit. In a split second the electrons drop right back down into their original orbit. When that happens, a new photon is thrown off by the atom.
5) That new photon now continues on it's way just as though it was the original photon.

Now even though the photon was moving at *exactly* 186,000 miles / second (called "C") when it hit the atom, and even though the new photon thrown off by the atom also moves at exactly 186,000 miles / second, it takes a brief instant for the photon to disappear, the electrons to move into a higher orbit, then drop back down to their original orbit, and a new photon to be thrown off.

So a good car analogy would go more like this:
The photon is like a car with a blind driver. As long as there's nothing to hit, fine, the photon just drives along at the speed of "C".
But when the photon starts to go through glass it's got all these glass molecules in the way. The photon always drives at exactly the same speed ("C"), but every time it runs into an atom, it has to stop for a second, exchange insurance information, and then continue on it's way (again traveling at exactly the speed "C"), until it hits ANOTHER atom, has to stop again to exchange insurance info with that one, before it can continue on again, until it hits yet ANOTHER atom...

So if you think of it THAT way it's easy to see why light moves through a vacuum more quickly than it moves through glass, even though the photon always, always always ALWAYS, moves at just one speed, which is 186,000 miles / second.
When a photon moves through vacuum it's got nothing to hit so it makes no stops.
But when a photon moves through glass it's CONSTANTLY running into things (atoms), so it makes a LOT of stops, so it take more time for it to make it's way through glass.

Make more sense?

So the bottom line here is that the photons, the "particles of light", are not like cars speeding up and slowing down in waves. The photons, (the cars), never ever speed up or slow down, not ever, not even a little bit. They all move at exactly the same speed, all the time, so they can't bunch up like cars.

I know exactly the problem you're having, because I had exactly the same problem with this when I was in 12th grade.
It bugs you out, TOTALLY, because you just can't wrap your head around the idea that photons just plane flat out aren't "like" anything at all in the "normal" world that you can picture in your head.
The plane fact of the matter is that "photons" are only "really like" photons, and "light waves" are only "really like" light waves. They're the same thing (photons and light waves), and they're different than anything in the "real world" that you can picture in your head.

So in a way all of us are like a blind man trying to picture in our heads what the color red looks like even though we can't see, and have never seen, the color "red".
"Red" light exists, it's very VERY real, it's not "just theory", but it's something that no matter how hard you try, if you're blind, you just can't imagine or picture what the color red is "really like" in your head.

The fact that light is made up of something that behaves like a particle AND a wave, all at the same time, is a bit like that. Just because we can't imagine in our heads exactly what it's "really like" doesn't change the fact that light behaves the way it does. In reality light is made of something that BEHAVES like it's made of particles (photons), and BEHAVES like it's made up of waves, but isn't really either of those things. It's made up of something that isn't like anything in our normal world that you can picture in your head any more than a blind guy can picture what the color red is "really like".

Frustrating I know, but you're going to find that there are a lot of things in science that are like this, things you can come to understand in that you can predict EXACTLY how they're going to behave (in particular with mathematics), but at the same time things that are nearly impossible to picture what they're "really like" in your head. Nature is chuck full of things that aren't "really like" anything at all that you can pick up and hold in your hand. But that doesn't stop you from figuring out how they behave, or how to make use of the behaviors you observe.

Science can in fact take you beyond the boundries of limited human imagination. All you have to do is learn to ... "Let go Luke".

 

[MonstersFromTheId]

I.e. "The cars are the photons and the waves of cars as they stop and start down the line are the wave portion so traffic is both wave and particulate."

And here's why - the particles (the cars) always always ALWAYS move at *exactly* the same speed (c = 186,000 miles / second). That never changes.
Even when light "slows down" as it moves through air or glass, the particles (photons) STILL move at exactly the same speed (c).

Now I know that sounds crazy, but it actually makes sense if you look at what happens when light moves through vacuum, as opposed to what happens when it moves through something like, say, glass.

When light moves through vacuum the photons have nothing at all to run into. So they just zip along, pefectly happy, at the ONLY speed they EVER move at, which is "C", the "speed of light in a vacuum", which is 186,000 miles /second.

But when light moves through glass the photons have PLENTY of things to run into. They have gazzillions of glass molecules to run into. And each and every time the photons run smack into one of the atoms of a glass molecule they have to take time to go through an energy exchange process.

1) The photon hits one of the atoms in one of the glass molecules.
2) The photon turns into energy, pure "kinetic" energy, as in the photon TOTALLY ceases to exist. For a split second there is no photon.
3) That energy boosts the electrons of the atom it hit into a higher orbit.
Picture a tether ball (an electron) swinging around a pole on a string (the "pole" being the nucleus of the atom). When the photon hits the electron it's like smacking the tether ball as it goes by to make it go around the pole even faster.
4) But the electrons won't stay in that higher orbit. In a split second the electrons drop right back down into their original orbit. When that happens, a new photon is thrown off by the atom.
5) That new photon now continues on it's way just as though it was the original photon.

Now even though the photon was moving at *exactly* 186,000 miles / second (called "C") when it hit the atom, and even though the new photon thrown off by the atom also moves at exactly 186,000 miles / second, it takes a brief instant for the photon to disappear, the electrons to move into a higher orbit, then drop back down to their original orbit, and a new photon to be thrown off.

So a good car analogy would go more like this:
The photon is like a car with a blind driver. As long as there's nothing to hit, fine, the photon just drives along at the speed of "C".
But when the photon starts to go through glass it's got all these glass molecules in the way. The photon always drives at exactly the same speed ("C"), but every time it runs into an atom, it has to stop for a second, exchange insurance information, and then continue on it's way (again traveling at exactly the speed "C"), until it hits ANOTHER atom, has to stop again to exchange insurance info with that one, before it can continue on again, until it hits yet ANOTHER atom...

So if you think of it THAT way it's easy to see why light moves through a vacuum more quickly than it moves through glass, even though the photon always, always always ALWAYS, moves at just one speed, which is 186,000 miles / second.
When a photon moves through vacuum it's got nothing to hit so it makes no stops.
But when a photon moves through glass it's CONSTANTLY running into things (atoms), so it makes a LOT of stops, so it takes more time for it to make its way through glass than it does to make its way through vacuum.

Make more sense?

So the bottom line here is that the photons, the "particles of light", are not like cars speeding up and slowing down in waves. The photons, (the cars), never ever speed up or slow down, not ever, not even a little bit. They all move at exactly the same speed, all the time, so they can't bunch up like cars.

I know exactly the problem you're having, because I had exactly the same problem with this when I was in 12th grade.
It bugs you out, TOTALLY, because you just can't wrap your head around the idea that photons just plane flat out aren't "like" anything at all in the "normal" world that you can picture in your head.
The plane fact of the matter is that "photons" are only "really like" photons, and "light waves" are only "really like" light waves. They're the same thing (photons and light waves), and they're different than anything in the "real world" that you can picture in your head.

So in a way all of us are like a blind man trying to picture in our heads what the color red looks like even though we can't see, and have never seen, the color "red".
"Red" light exists, it's very VERY real, it's not "just theory", but it's something that no matter how hard you try, if you're blind, you just can't imagine or picture what the color red is "really like" in your head.

The fact that light is made up of something that behaves like a particle AND a wave, all at the same time, is a bit like that. Just because we can't imagine in our heads exactly what it's "really like" doesn't change the fact that light behaves the way it does. In reality light is made of something that BEHAVES like it's made of particles (photons), and BEHAVES like it's made up of waves, but isn't really either of those things. It's made up of something that isn't like anything in our normal world that you can picture in your head any more than a blind guy can picture what the color red is "really like".

Frustrating I know, but you're going to find that there are a lot of things in science that are like this, things you can come to understand in that you can predict EXACTLY how they're going to behave (in particular with mathematics), but at the same time things that are nearly impossible to picture what they're "really like" in your head. Nature is chuck full of things that aren't "really like" anything at all that you can pick up and hold in your hand. But that doesn't stop you from figuring out how they behave, or how to make use of the behaviors you observe.

Science can in fact take you beyond the boundries of limited human imagination. All you have to do is learn to ... "Let go Luke".

 

[Mau]

That was an excellent explanation MonstersFromTheId!

Just to clarify: the photons really have 2 speeds: c and 0?

 

[blimkie]

look up wave particle duality

 

[FreeMatter]

look up wave particle duality

Do you mean there is a contradiction?

 

[alias25]

I thought light was a particle and the probability of this particle being at a certain place and time gave it a wave like behaviour...

 

[TheAntiRelative]

Doesn't quantum uncertainty make it where they are not necessarily in anyone place at anyone time but basically to our peception because we tested it, it seems so. Or something like that...

I mean, a single photon cannot really be constrained by time as we are otherwise it's speed could not be constant. In the infinite frames of reference, if a single photon was somhow tagged, observers from those infinite frames would believe it to be "simultaneously" in infinite places from emission to absorption.

Basically the non-existance of simultaneity precludes a photon from truly being in one place at one time doesn't it?

 

[ZapperZ]

Now I know that sounds crazy, but it actually makes sense if you look at what happens when light moves through vacuum, as opposed to what happens when it moves through something like, say, glass.

When light moves through vacuum the photons have nothing at all to run into. So they just zip along, pefectly happy, at the ONLY speed they EVER move at, which is "C", the "speed of light in a vacuum", which is 186,000 miles /second.

But when light moves through glass the photons have PLENTY of things to run into. They have gazzillions of glass molecules to run into. And each and every time the photons run smack into one of the atoms of a glass molecule they have to take time to go through an energy exchange process.

1) The photon hits one of the atoms in one of the glass molecules.
2) The photon turns into energy, pure "kinetic" energy, as in the photon TOTALLY ceases to exist. For a split second there is no photon.
3) That energy boosts the electrons of the atom it hit into a higher orbit.
Picture a tether ball (an electron) swinging around a pole on a string (the "pole" being the nucleus of the atom). When the photon hits the electron it's like smacking the tether ball as it goes by to make it go around the pole even faster.
4) But the electrons won't stay in that higher orbit. In a split second the electrons drop right back down into their original orbit. When that happens, a new photon is thrown off by the atom.
5) That new photon now continues on it's way just as though it was the original photon.

Now even though the photon was moving at *exactly* 186,000 miles / second (called "C") when it hit the atom, and even though the new photon thrown off by the atom also moves at exactly 186,000 miles / second, it takes a brief instant for the photon to disappear, the electrons to move into a higher orbit, then drop back down to their original orbit, and a new photon to be thrown off.

I was going to let this go since this has been explained several times, but I think if I do that, the misconceptionn will continue. So I have to jump in here.

The problem with this explanation is that the "transition" being done here is not "atomic". When things are formed in a solid, most of the property of that material do not exactly reflect the property of the individual atom. The atoms have lost a lot of their individual property and now become a "collective". How they form the solid, how they are arranged, etc. now are more important. It is why you can get an arrangement of carbon atoms one way and get graphite, while another arrangement will give you diamond, two very DIFFERENT material with different properties. Yet, both are made up of carbon atoms!

Optical conductivity (i.e. how EM wave or photons goes through solid) is very complex and depends on something very important - the phonon or vibrational mode of the solid. EM wave has an oscillating E-field. Solids have charge ions that make up its structure. When a photon hits a solid, several things can happen:

1. The photon encounters a bunch of conduction electrons, as in a metal. The conduction electrons absorbs the photon, and oscillate at the same frequency as the incoming photons. This will cause the electrons to re-radiate the photon. With conservation of momentum laws and the recoil of the lattice ions, this will give you the reflection laws that we are all familiar with.

2. The photon encounters a material such as glass, which is transparent in the visible range. Here, what happens is that the light is temporarily absobed by the lattice vibration, BUT, because the phonon or vibrational mode is not available, this energy cannot be absorbed. It's like trying to oscillate something but not at its natural resonance frequency - it just won't want to do it. So this energy is spit back out by the ion motion and thus, the photon gets retransmitted. This goes on till the photon escapes on the other side.

3. The photon encounters a material in which the phonon mode IS available and thus, it is absorbed and turned into a lattice vibration. This is HEAT! Thus, the material is opque to this particular photon.

Moral of the story - in optical conductivity through solid, there is usually NO "atomic transition". It is the solid band structure, i.e. the collective behavior of ALL the atoms in the solid, that is now relevant.

Zz.

 

[MonstersFromTheId]

GREAT explanation (and some q's)

"Moral of the story - in optical conductivity through solid, there is usually NO "atomic transition". It is the solid band structure, i.e. the collective behavior of ALL the atoms in the solid, that is now relevant."
When you say "ALL" the atoms in the solid, do you mean "ALL" the atoms as in all *types* of atoms in a structure (i.e. you need a representative minimum number of atoms to determine the collective behavior of the rest of the solid)?
Or by "ALL" atoms in the solid, do you mean literally ALL the atoms that are part of the solid.
The reason I ask.
I could see where it wouldn't take very much time at all for the effects of a photon hitting a solid to spread out through a representative minimum number of atoms, so that the collective properties of the rest of the solid could be established (i.e. the photon is reflected, absorbed, and/or transmitted on the basis of the behavior of that representative minimum number of atoms that's typical of the rest of the solid).
But if ALL the atoms that are part of the solid are required to determine whether or not the photon is reflected, absorbed, and/or transmitted, then I could see where the overall size of the solid would start to effect how long it takes before the photon actually IS reflected, absorbed, and/or transmitted.
Second question:
When talking about transparent solids, it would be a bit misleading to think of photons as traveling *through* the solid? It's more like photons hit one side of a piece of glass, the energy can't be absorbed, so the energy bleeds back out the other side of the glass as essentially new photons?

 

[DaTario]

I believe the so called wave behavior of light does not have, stricly speaking, a close relation to what we understand as wave.

Wave is a manifestation, a collective behavior which presents properties such as propagation and interference. But it is fundamentally populational, i.e., wave needs more than one particle to exist. It needs a medium, either discrete or continuous system of particles. Light presents propagation and interference as well, but I refuse to put it as an ordinary wave.

Waves can start anywhere, while light can only start at charges.

We may try to shed some light on this concept, for the present situation of photon understanding seems to be rather unconfortable and perhaps ridiculous.

Best Regards,

DaTario

 

[TheAntiRelative]

ZapperZ:

So you are saying that transparency is actually linked with the resonant frequencies / modes / etc of a solid?

 

[ZapperZ]

When you say "ALL" the atoms in the solid, do you mean "ALL" the atoms as in all *types* of atoms in a structure (i.e. you need a representative minimum number of atoms to determine the collective behavior of the rest of the solid)?
Or by "ALL" atoms in the solid, do you mean literally ALL the atoms that are part of the solid.
The reason I ask.
I could see where it wouldn't take very much time at all for the effects of a photon hitting a solid to spread out through a representative minimum number of atoms, so that the collective properties of the rest of the solid could be established (i.e. the photon is reflected, absorbed, and/or transmitted on the basis of the behavior of that representative minimum number of atoms that's typical of the rest of the solid).
But if ALL the atoms that are part of the solid are required to determine whether or not the photon is reflected, absorbed, and/or transmitted, then I could see where the overall size of the solid would start to effect how long it takes before the photon actually IS reflected, absorbed, and/or transmitted.
Second question:
When talking about transparent solids, it would be a bit misleading to think of photons as traveling *through* the solid? It's more like photons hit one side of a piece of glass, the energy can't be absorbed, so the energy bleeds back out the other side of the glass as essentially new photons?

If you take the atoms (all kinds of them) apart, you will not have any phonons. It is the reason why phonons are called "collective" excitation. The lattice ions and the way they are arranged determine what kind of phonon spectrum is available for that particular solid.

Now the confusion comes in as in the SIZE of such domain. Note that for something the order of microns is already large when compared to the individual size of atoms and ions in the solid. This is sufficient for many-body physics to kick in.

There is no definite transition between when the number is too small for phonons to be present. The study in the mesoscopic regime is still an ongoing research area.

As for transparency, if you don't care about the details, then it is perfectly fine to think about the SAME photon emerging on the other side. But if "god is in the details", then that picture will have problems, not the least of which is the indistinguishable statistics of bosons.

Zz.

 

[daniel_i_l]

light as particle

I find this problem easier to swallow when I think about the fact that really, photons arn't so special. All matter (electrons , humans , planets...) also has wavelike properties! But in the macroscopic world these properties aren't as noticable.
It helps if you think about photons (and all matter) as particles that move according to their "wave function". In the quantum world, it is impossible to predict were a particle will be in a certain time. Rather you can predict the probibiliy of a particle to be at a certain spot.
This is were the wave comes in. Where there is more chance for the particle to be found in a certain place, the value of the wave there is bigger, were there is less chance the value of the wave is smaller. You can think about the "wave" part of a particle as a probibility wave that "says" how much chance you have of finding the particle at a certain location. But really the photon is only a particle. You get interferance when the probibility wave cancels out at certain places and goes to zero. That means that no photons have a chance of going there, and you get black lines.