B What is light exactly? Electromagnetic wave or photons?

1. Dec 29, 2017

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 .

2. Dec 29, 2017

PeroK

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.

3. Dec 29, 2017

Staff: Mentor

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.

4. Dec 29, 2017

ISamson

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

5. Dec 29, 2017

davenn

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

6. Dec 30, 2017

Mister T

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.

7. Dec 30, 2017

phinds

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.

8. Dec 30, 2017

Mister T

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.

9. Dec 30, 2017

phinds

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.

10. Dec 30, 2017

Mister T

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.

11. Dec 30, 2017

_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.

12. Dec 30, 2017

Daniel Sellers

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.

13. Dec 31, 2017

rcgldr

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)?

14. Dec 31, 2017

Daniel Sellers

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.

15. Dec 31, 2017

PeroK

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

That would be the quantum interference of probability amplitudes.

16. Dec 31, 2017

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.

17. Dec 31, 2017

Staff: Mentor

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.

18. Dec 31, 2017

Mister T

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

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.

19. Dec 31, 2017

sophiecentaur

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.

20. Dec 31, 2017

Drakkith

Staff Emeritus
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.