What is light exactly? Electromagnetic wave or photons?

[tor1990]

When why sey that light is electromagnetic wave i understand this. But what i do not understand is where photons come into picture ? can somebody explain me the relationship between those two .

My knowledge in this respect is not great so I would ask that the answers be adjusted .

 

[PeroK]

When why sey that light is electromagnetic wave i understand this. But what i do not understand is where photons come into picture ? can somebody explain me the relationship between those two .

My knowledge in this respect is not great so I would ask that the answers be adjusted .

The simple answer is that light delivers energy in discrete quanta, called photons. Look up the "photoelectric effect".

The energy of a photon is related to the frequency of the wave.

 

[Dale]

When why sey that light is electromagnetic wave i understand this. But what i do not understand is where photons come into picture ? can somebody explain me the relationship between those two .

My knowledge in this respect is not great so I would ask that the answers be adjusted .

There are basically three theories of electromagnetism:

Circuit theory
Maxwell’s equations
Quantum electrodynamics

Circuit theory ignores EM radiation entirely. Maxwell’s equations deals with EM radiation as waves in a classical EM field. Photons are only involved in QED. You only need them to explain the interaction of EM fields and matter.

Honestly, at a B level, I would recommend you stay away from photons. QED is not a trivial topic and most simplifications will be misunderstood without the framework of QED.

 

[ISamson]

The simple answer is that light delivers energy in discrete quanta, called photons. Look up the "photoelectric effect".

The energy of a photon is related to the frequency of the wave.

Could I understand this as "light comes in discrete packets of EM wave, which we call 'photons'?

 

[davenn]

Could I understand this as "light comes in discrete packets of EM wave, which we call 'photons'?

the EM wave is continuous, photons are the energy carriers in the wave

 

[Mister T]

Could I understand this as "light comes in discrete packets of EM wave, which we call 'photons'?

No I don't think so, although some might disagree with me.

If you imagine a source that emits light of a single color (monochromatic) and try to lower the intensity of the light, you will find that there is a lowest nonzero amount of light, called a quantum, or a photon. If you then increase the intensity you find that you can do it only in multiples of that one smallest amount.

If you have lots and lots of photons in a beam of light only then does the electromagnetic wave nature become apparent.

The electromagnetic wave is an approximation, valid only for large numbers of photons.

 

[phinds]

If you have lots and lots of photons in a beam of light only then does the electromagnetic wave nature become apparent.

That goes against the results of the double slit experiment when single photons are fired one after the other, which clearly shows that single photons exhibit wave nature.

 

[Mister T]

That goes against the results of the double slit experiment when single photons are fired one after the other, which clearly shows that single photons exhibit wave nature.

By "exhibit wave nature" I assume you mean the appearance of the interference pattern? But note that to have the interference pattern emerge you must fire many many more photons, whether you fire them one at a time or not.

 

[phinds]

By "exhibit wave nature" I assume you mean the appearance of the interference pattern? But note that to have the interference pattern emerge you must fire many many more photons, whether you fire them one at a time or not.

But the point is that they are fired one at a time and as such exhibit wave behavior, which contradicts your statement. The fact that you have to do a lot of them to see a visible interference pattern is irrelevant to the point at hand.

 

[Mister T]

But the point is that they are fired one at a time and as such exhibit wave behavior, which contradicts your statement. The fact that you have to do a lot of them to see a visible interference pattern is irrelevant to the point at hand.

I suppose that depends on what you mean by wave behavior. If you mean the appearance of an interference pattern, and you cannot get that by firing a single photon through a double-slit apparatus, then you cannot exhibit wave behavior with a single photon.

 

[_PJ_]

Considering a photon in terms of a localised (position) with superposition of momenta, then the wavelike (as in, follows the mathematics of wavelike periodic phenomena) nature is still inherent in how those multiple frequencies are combined.

There may be ways to consider light in more localised, or with a potentially infinite extent - it really depends on the experiment or model you are using and what it is you're trying to calculate/demonstrate at the time.

Light itself is neither a dot, a point, a small packet of waves, a wave or any such thing. It's instead "electromagnetic energy" and this behaves according to rules as defined as those of excitations in electromagnetic fields (at least as far as current standard model "particle" physics )

Quantum is descriptive as there are limits to the energies that might be emitted or absorbed in discrete terms.
"Particle" is historical from days when things used to be really thought as tiny pieces. The term is still used when considering discreteness, but should not be interpreted as 'dots', little balls or certainly not 'points'.

That's my understanding at least.

 

[Daniel Sellers]

I suppose that depends on what you mean by wave behavior. If you mean the appearance of an interference pattern, and you cannot get that by firing a single photon through a double-slit apparatus, then you cannot exhibit wave behavior with a single photon.

You can observe wave behavior by noting that the single photon is detected at a position where the mathematical interference pattern tells you it is likely to be, and that you have certainly not detected at a position of maximum destructive interference. Indeed, to actually see the image on the screen requires a great many photons, but each individual photon 'behaves' in a wavelike manner.

How many photons do you need to fire at the screen before you see the interference pattern? Perhaps you want to see 'many, many' recorded detections, but the wave behavior is observable in each, individual photon nonetheless.

EDIT: The OP's question is certainly not easy to answer in simple terms. I happen to have been reading about foundations of quantum theory over this Winter break and from that vantage point there seems to be little consensus on the 'real, physical' nature of quantum particles such as photons.

 

[rcgldr]

That goes against the results of the double slit experiment when single photons are fired one after the other, which clearly shows that single photons exhibit wave nature.

By "exhibit wave nature" I assume you mean the appearance of the interference pattern? But note that to have the interference pattern emerge you must fire many many more photons, whether you fire them one at a time or not.

But the point is that they are fired one at a time and as such exhibit wave behavior, which contradicts your statement. The fact that you have to do a lot of them to see a visible interference pattern is irrelevant to the point at hand.

What would happen if a 1000 experiments were run in parallel, each experiment firing a single photon through a double slit, and noting the relative (to each experiment) location of impact of the photon? Would there be an interference like distribution in the 1000 experiments? What evidence is there to imply that an interference like distribution is due to wave like behavior as opposed to some other type of interaction between the double slit and photons (perhaps some type of photon capture and release that may or may not slightly change the angle of a photon)?

 

[Daniel Sellers]

What would happen if a 1000 experiments were run in parallel, each experiment firing a single photon through a double slit, and noting the relative (to each experiment) location of impact of the photon? Would there be an interference like distribution in the 1000 experiments? What evidence is there to imply that an interference like distribution is due to wave like behavior as opposed to some other type of interaction between the double slit and photons (perhaps some type of photon capture and release that may or may not slightly change the angle of a photon)?

If you did 1000 identical experiments and superposed the resulting detections, yes they would show an interference patter with 1000 dots.

What evidence is there that this interference pattern is due to wave behavior? Well for one thing, no phenomena in nature except waves exhibit such interference.

There is other evidence as well. By imagining that electrons exhibit wave behavior, de Broglie predicted that one could produce an interference pattern by firing an electron beam through a crystal lattice. He was correct.

Basically the term 'wavelike behavior' is a useful qualitative description. If you want to theorize some other mechanism that perfectly mimics the results predicted by wave interactions then by all means, have fun.

 

[PeroK]

What evidence is there that this interference pattern is due to wave behavior? Well for one thing, no phenomena in nature except waves exhibit such interference.

Particles may exhibit an interference pattern. You said it yourself, in fact:

There is other evidence as well. By imagining that electrons exhibit wave behavior, de Broglie predicted that one could produce an interference pattern by firing an electron beam through a crystal lattice. He was correct.

Basically the term 'wavelike behavior' is a useful qualitative description. If you want to theorize some other mechanism that perfectly mimics the results predicted by wave interactions then by all means, have fun.

That would be the quantum interference of probability amplitudes.

 

[Daniel Sellers]

Yes, I am saying that particles exhibit wavelike behavior, that was my point. The question I was responding to is 'what evidence is there that the interference pattern is due to wave behavior?'

Perhaps I should have specified that no macroscopic phenomena except for waves interfere in that way. So we call the interference observed with quantum particles 'wavelike.'

Also, I am a relative newbie to QM, but isn't the interference of probability amplitudes the same thing? Granted, the wave function for a quantum particle involves complex numbers and may or may not (depending on your interpretive bent) have anything to do with a wave in real 3-space, but the 'interference of probability amplitudes' could still be aptly described as wavelike.

 

[Dale]

What evidence is there to imply that an interference like distribution is due to wave like behavior as opposed to some other type of interaction between the double slit and photons (perhaps some type of photon capture and release that may or may not slightly change the angle of a photon)?

The question isn’t really if it is wavelike or not. The question is if a single particle goes through a single slit, and the answer is that under certain conditions a single particle goes through both slits! Whether this is wavelike or not, it is different from classical mechanics.

 

[Mister T]

What would happen if a 1000 experiments were run in parallel, each experiment firing a single photon through a double slit, and noting the relative (to each experiment) location of impact of the photon?

I don't see how that's any different than sending 1000 photons through the same apparatus.

What evidence is there to imply that an interference like distribution is due to wave like behavior as opposed to some other type of interaction between the double slit and photons (perhaps some type of photon capture and release that may or may not slightly change the angle of a photon)?

It's not "due to" any wave-like behavior. It's explained as wave-like behavior. As far as I know there is no explanation of the type you suggest.

Whenever you see the word "theory" you can replace it with "explanation". The fact is that there may be other explanations of any phenomenon, and often there are, but there will never be any "evidence" that these other explanations exist. Evidence is a display of Nature's behavior, explanations are inventions of the human mind.

 

[sophiecentaur]

it is different from classical mechanics.

People just don't believe this. They say "yes but . . . . . . ." when they really have to accept that the classical model does not answer the questions.
This is not to say that the present theories are necessarily the end of things; they are definitely not.

 

[Drakkith]

People just don't believe this. They say "yes but . . . . . . ." when they really have to accept that the classical model does not answer the questions.
This is not to say that the present theories are necessarily the end of things; they are definitely not.

Indeed. Regardless of what future models/theories say, or what "the truth" actually is, classical physics does not and cannot accurately describe/explain/predict the results from the double slit experiment. Just like how Newton's Theory of Universal Gravitation does not and cannot accurately explain the precession of Mercury or the difference in clock rates at different heights in a gravitational field.

 

[Nugatory]

How many photons do you need to fire at the screen before you see the interference pattern? Perhaps you want to see 'many, many' recorded detections, but the wave behavior is observable in each, individual photon nonetheless.

It is not. You fire one photon at the screen and one dot appears; this behavior is exactly that of an indisputably particle-like rifle bullet.

The "interference pattern" prediction of quantum mechanics is probabilistic, and it is impossible to observe any probabilistic phenomenon with a single observation.

 

[lekh2003]

By "exhibit wave nature" I assume you mean the appearance of the interference pattern? But note that to have the interference pattern emerge you must fire many many more photons, whether you fire them one at a time or not.

I have to disagree with most of what you are saying. In this situation, you cannot say that as you add more photons, light becomes more like a wave. Not only is that a confusing statement, it is near incorrect. Light is always a wave, just that you can't really see it sometimes.

What would happen if a 1000 experiments were run in parallel, each experiment firing a single photon through a double slit, and noting the relative (to each experiment) location of impact of the photon? Would there be an interference like distribution in the 1000 experiments?

Even if you shoot singular photons in completely separate experiments, you will see a distribution pattern.

I don't see how that's any different than sending 1000 photons through the same apparatus.

It isn't different. It is exactly like sending it through one apparatus just one at a time. The point of the experiment is that the wave nature is present at every instance, be it one photon or a thousand.

 

[PeroK]

Also, I am a relative newbie to QM, but isn't the interference of probability amplitudes the same thing? Granted, the wave function for a quantum particle involves complex numbers and may or may not (depending on your interpretive bent) have anything to do with a wave in real 3-space, but the 'interference of probability amplitudes' could still be aptly described as wavelike.

It's not the same thing. The electron is a (quantum) particle whose dynamic properties (position, momentum, energy, angular momentum, spin angular momentum) are governed by a "probability amplitude" function. This is usually called the "wave" function, but that is a coincidence of terminology that leads some people to get confused about whether it is "real" wave or not.

In any case, you can (and should, in my opinion) learn QM without any reference to the wave-particle duality or "wavelike" behaviour of particles. I have two QM books:

In Griffiths, wave-particle duality is mentioned once, as a historical footnote on page 420. And, in Sakurai it doesn't get a mention at all.

Also, if you are new to QM, I highly recommend Feynman's Messenger lecture:

http://www.cornell.edu/video/richard-feynman-messenger-lecture-6-probability-uncertainty-quantum-mechanical-view-nature

And, especially, how he talks about "waves" and "particles".

 

[Mister T]

I have to disagree with most of what you are saying. In this situation, you cannot say that as you add more photons, light becomes more like a wave.

The OP's question was about electromagnetic waves. Those waves are solutions to Maxwell's Equations and as such completely ignore quantum mechanics. Photons are purely quantum mechanical particles, and in quantum theory Maxwell's Equations are an approximation, valid only in the limit of large numbers of photons.

Thus light as an electromagnetic wave is a picture that appears only when large numbers of photons are involved.

 

[ZapperZ]

I have to disagree with most of what you are saying. In this situation, you cannot say that as you add more photons, light becomes more like a wave. Not only is that a confusing statement, it is near incorrect. Light is always a wave, just that you can't really see it sometimes.Even if you shoot singular photons in completely separate experiments, you will see a distribution pattern.

It isn't different. It is exactly like sending it through one apparatus just one at a time. The point of the experiment is that the wave nature is present at every instance, be it one photon or a thousand.

This entire post is completely wrong.

The which-way experiment clearly shows that light does NOT behave as a classical wave. The anti-bunching experiments clearly show that light does NOT behave as a classical wave.

Mister T is correct, that we tend to detect more and more of the classical wave nature when we are dealing with large number of photons, and when we are dealing with longer and longer wavelengths. So yes, there ARE regimes where the classical wave nature of light is INSUFFICIENT to describe what we observe.

If you have the classical wave description to describe those two types of experiments that I mentioned above, I'd like to see it. Otherwise, you need to think twice before you spew such wrong information.

Zz.

 

[lekh2003]

The OP's question was about electromagnetic waves. Those waves are solutions to Maxwell's Equations and as such completely ignore quantum mechanics. Photons are purely quantum mechanical particles, and in quantum theory Maxwell's Equations are an approximation, valid only in the limit of large numbers of photons.

Thus light as an electromagnetic wave is a picture that appears only when large numbers of photons are involved.

Ok @Mister T, thanks for clarifying my post. I was thinking of a quantum mechanical perspective but I seemed to have been missing the point of the OP's question.

I'll revise my answers further next time.