## Electromagnetic waves question

Started by 4 months ago15 replieslatest reply 4 months ago174 views

Hi All,

This question suddenly popped up in my head.

Why is that nobody talks about electromagnetic wave interaction as they share the same space?

Yes they are different frequencies but surely some sections can be in-phase or anti-phase. I know the double slit experiment that led to concept of quantum physics talks about phase of light waveforms.

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Reply by May 15, 2023

I don't know if I am understanding your question correctly, but in fact everybody talks about the interaction of electromagnetic waves. Maxwell's theory is linear, which leads to the superposition of waves when they interfere. This adds or subtracts the amplitude components according to their relative phase. That is why when you turn on say 4 light bulbs in your room, you do not see it 4 times as bright. (Similarly, in sound, 4 violins do not sound 4 times as loud as one violin.)

When frequencies are very different, this superposition leads to very interesting phenomena, like frequency modulation for example, used in oscillators. (In sound, this leads to many of the sound effects that we so much enjoy.)

The double-slit experiment, that you mention, is also a superposition of waves passing through each of the slits. The problem there is different, because as you lower the intensity to be that of individual photons, then one photon at a time hits the screen but nevertheless you get an interference pattern, which is weird. The mathematics of this are embedded in quantum theory, which is also a linear theory and the principle of superposition applies.

It is the interaction of light with matter which is much more difficult, both at the classical and quantum levels. If you are interested in this I can gladly supply references.

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Reply by May 15, 2023

Thanks for the clear reply. Your example of room light intensity is intuitive.

I was thinking of old short wave radio reception when signal was varying in amplitude and explained on basis of multipath (ionosphere delay effect).

Then I thought of our normal day/night visibility of objects. With so many multiple light sources reaching the eye yet we see it as perfectly stable intensity as long as sources are stable, even when we move around. I wonder if this is expected physics-wise or it is adaptation.

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Reply by May 15, 2023
Hi Kaz,  I tried to reply to your first question, but for some reason it was not posted. The

replies regarding the interference of different light waves are correct of course. But maybe you meant something else. If by "interaction" you mean that "do light waves exchange energy/momentum such that they influence each other dynamically?", well then of course they can. There are two ways this can happen. First, large intensity light waves in non-linear optical materials can mix with each other to produce new light frequencies. For example, the process of sum frequency generation mixes say IR radiation with red light to produce blue light, there are many examples.  Please see this link https://en.wikipedia.org/wiki/Sum-frequency_genera...  The second effect is a quantum effect and occurs in free space, not requiring non-linear materials. When high energy electromagnetic radiation, such as gamma ray photons collide with each other. They can exchange energy and momentum to produce massive particles. Thus matter, can be created from pure electromagnetic radiation. This process is called gamma-gamma scattering, you can read more about it here https://en.wikipedia.org/wiki/Two-photon_physics.

I hope I helped shed some light on the matter (please forgive pun)

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Reply by May 15, 2023

Thanks Safwan for the interesting examples.

Coming down to a simpler case. Imagine when eye receives white light reflected from a surface. Obviously it includes all visible spectrum. We know that for pure white, at eye level, all frequencies have same amplitude at any distance even though phases differ along the distance.

I could be understanding laws of physics wrongly but it would have made more sense to me if some colour change effect (like a rainbow) is caused by distance change.

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Reply by May 15, 2023

The sensation of colour as decoded by the cone cells in the retina depends on the light photon energy, which is a quantum mechanical effect. The photon gives up its energy to the cells, which in turn send corresponding modulated nerve impulses to the visual cortex in the brain. An ill-understood psycho-visual process takes place resulting in colour perception. So, in answer to your question, even though the colour-defining photon energy is related to the wavelength of light, it has nothing to do with the phase. The phase tell us the degree of coherence in the light, i.e. how many photons arrive simultaneously at the retina, and therefore decides the light intensity as dim  vs. bright. Under normal circumstances, unlike the special cases I mentioned in my earlier post, photons neither interact with each other, nor do they spontaneously change their wavelength or energy - regardless of the distance travelled. We know the composition of stars by the colour of the light they emit. Although colour changes with the speed of the emitter (Doppler shift/Red shift), colour does not change with distance itself. And stars are very far indeed.

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Reply by May 15, 2023

So the eye sees photons (not waves). That implies we got two models. I wonder why RF engineers haven't tried along the same concept of using photons instead of waves and MIMO technology. But it is way too early to understand functionality of brain.

Many thanks.

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Reply by May 15, 2023

You're right. RF engineers have no need for photon processing. At these frequencies, and with the very large numbers of photons involved, the RF electromagnetic radiation behaves much more like waves than particles. Moreover, the electrons that detect the RF energy in antenna and circuits are better modelled as responding to the electric field in the RF wave, rather than to single very-low-energy RF photons.

But all other communication engineers using fibre optics, optical storage and computing, most definitely deal with the photon aspect of light.

Take care

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Reply by May 15, 2023
"Thanks for the clear reply. Your example of room light intensity is intuitive."

And it's false.  That kind of room light is not coherent.

The second part is simply time averaging of the sensors.  Fluorescent lights bother me by their shimmering.  Others say they don't get that.

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Reply by May 15, 2023

Thanks Cedron for your viewpoint.

May be we need to do more tests to prove such intricate concepts.

By the way I am fascinated by double slit experiment but not sure how do they observe a single photon. Does the sensor looks at the photon with another photon? wouldn't such observing technique impact the result?

Kaz

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Reply by May 15, 2023
https://en.wikipedia.org/wiki/Double-slit_experime...

"In the basic version of this experiment, a coherent light source, such as a laser beam, illuminates a plate pierced by two parallel slits, and the light passing through the slits is observed on a screen behind the plate ... However, such experiments demonstrate that particles do not form the interference pattern if one detects which slit they pass through. These results demonstrate the principle of wave–particle duality."

That's standard stuff, not just my viewpoint.  Where my viewpoint differs from standard is I don't believe photons are carriers of EM, they are different things entirely and just happen to travel at really close to the same speed due to a dependency.

I can also clearly state that the derivation of Maxwell's Law presumes a constant speed of propagation, which does not occur in reality.  This isn't the forum to go into detail.

As far as the other issues, about sensors, you are in essence talking about Goethe vs Newton, perception vs reality.  Again, I don't feel like delving into detail, you can find it as easily as I have.  Light/matter interaction is a very hot topic right now with a lot of advances in photonics.
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Reply by May 15, 2023

Well, people talk about it all the time.  But often not in a way you might recognize.  Any antenna that is not an omnidirectional radiator, which is EVERY practical antenna, depends on (or suffers from) the interaction of electromagnetic waves.  Phased arrays of antennas are a great example -- steer a radio wave based on the phasing of many antennas.

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Reply by May 15, 2023

Who says nobody talks about it?   It's why we have spatial and frequency nulls due to multipath, and whole bunch of other effects.

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Reply by May 15, 2023

Long time ago (45 years), I took a class in antenna design and it was like an analog signal processing class.  Like using Tchebyshev polynomials to design antenna arrays that minimized the sidelobes.  I learned about them at about the time I learned about Tchebyshev filters.  Similar math.

EM is hard and mathy.  But I was into audio and never went that direction.

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Reply by May 15, 2023

... I use the DFT to synthesize antenna patterns in multi-element arrays for this reason. I love it when disciplines overlap to the same underlying principles!

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Reply by May 15, 2023

Hi Kaz, The replies already given are correct answers, but maybe you mean something else. If you mean by "interaction" that the waves should exchange energy/momentum: Well, they do!  However, because of the lack of a rest mass for the "normal" electromagnetic waves, the time average of the oscillating wave interaction is zero. However, if the energies of the photons making up the electromagnetic waves is high enough, then the photons collide with each other like billiard balls producing other massive particles. This is called gamma-gamma scattering.  You can read this here https://en.wikipedia.org/wiki/Two-photon_physics.