Light & Coherence part 1: Temporal Coherence

This is the first episode about coherence and how this wave phenomenon can cause waves to behave like localized entities.
Unfortunately the quantum (or corpuscular) description of light leads to a lot of confusion. The goal of this video is to describe "quantized" behavior of light purely from wave principles.
Contents:
0:00 Intro
1:04 Historical perspective
2:58 Quantization and the photoelectric effect
6:42 Light is just waves
7:24 Coherence explained
12:54 Temporal coherence as a sum of EM-fields
16:47 Coherence length vs. spectral band width
20:25 experiments on the coherence length of light
Link to the referenced Physics Explained video:
• What is the Heisenberg...
A sharp viewer noted that there is an error in the formula on the sheet at 27:25 : The product of delta time and delta energy in the Heisenberg uncertainty principle is not equal to (h*4*pi) but h/(4*pi), so a much smaller value. This also makes the value of delta frequency times delta time 1/(4*pi) not 4*pi Unfortunately I did not double check the values. Thanks for pointing this out Steve.
In this video short clips of other KZbin channels were used for illustration. Because of their short length and purpose, they are to be considered to be "fair use".
@TheActionLab
@TechIngredients
@ArvinAsh
@pbsspacetime
/ veritasium
@upandatom
@ProfessorDaveExplains
/ lookingglassuniverse
Did I forget anyone? Please let me know and I'll set things straight.
End music: Floating; The Early Birds. © JJM Vleggaar, 1999

[@gawayne1374]

One of my favourite things about this channel, is that it doesn't dumb things down. It's very well explained! But it doesn't rely on mediocre metaphors to do so.

[@amarissimus29]

The tragedy our our time is our belief that knowledge is the same as understanding a metaphor.

[@aniksamiurrahman6365]

Very well said.

[@VincentGroenewold]

Exactly this! Back in highschool I always looked in higher level books to understand stuff, just saying "accept this and learn it by heart" didn't work for me. :)

[@simonlinser8286]

i agree. i know there's kids and people who don't know, but sometimes it's likesometimes they start off everything as if we are in first grade, which is great, but sometimes it's like there's 5 minutes of actual discussion of the topic and the other 30 minutes is an introduction just to be able to talk about it. ive heard about the double slit experiment like 50 times just on KZbin, more than i ever learned about it in school or before KZbin physics videos were a thing...

[@harriehausenman8623]

@@amarissimus29 wow. that was deep.

[@freddytime]

The introduction of this video shows most of the channels I watch the most. And then there is yours, every time showing contents on a different new level, easier to grasp but somehow illuminating. I can’t thank you enough for producing this outstanding content!

[@kirkhamandy]

I also watch these other channels but, apart from Physics Explained, I find them useful introductions to something I may wish to go and learn more about elsewhere. That's where this channel (and, again, Physics Explained) differ, they both offer the same intro but just go _that extra mile_ (sorry, I'm not very good with words) into the subject and at just the right level. There's just _more to it_ then the 8-minute wonder videos. Which is why I guess they're longer than 8 minutes, lol.

[@simonlinser8286]

maybe it comes with age...

[@keesnuyt8365]

Well said: illuminating

[@MadScientist267]

There are a handful of them that are absolutely useless. This dude here explains things much better. Leave the "science guy" to "nye" lol... Dorky presentations like action lab are half the problem with people's understanding.

[@SplendidKunoichi]

man, i remember not even caring about any of this sort of stuff that all these other channels would get into until after i discovered this one

[@tenns]

i took an optics course in college because of you! thanks for the videos!

[@itishappy]

hell yeah brother!

[@tissuepaper9962]

Same, lol. Shared the link around the class, too.

[@danteallucinante]

Don't WASTE your time in college m8. Do it properly on your own if you want ANY actual success in life :)

[@tenns]

@@danteallucinante tell me you've never studied physics at a college level without telling you've never studied physics at a college level; Also very american way of thinking, college is pretty much free all around the world, you base yourself on a shit premise I encourage people to go to college unlike this dumbass

[@Beregorn88]

That was a trip down the memory lane! During my master degree in phisics, my research group developed a self-referencing interferometric method to measure spatial and temporal coherence of light beams. It uses the interference between the field scattered by weakly interactive particles (a colloidal suspension of polystirene nanoparticles in water) and the uneffected field: each nanoparticle emits a weak spherical wave that produce a pattern of circular fringes, whose visibility contains the information about the coherence of the light beam. All the circular patterns combine to form a speckle field, but since the scattered intensity is small compared to the incident beam, you can ignore the second order interactions and it can be demonstrated that the 2D spatial autocorrelation of the speckle field provide a sort of "average" of all the single patterns, allowing you to extract the information on the coherence with a very good signal to noise ratio. As a bonus, since all the nanoparticles move randomly of brawnian motion, if you use the difference of two picture taken at different times you can remove all the source of static noise (defect on the cell, dust on the optics, etc) retaining all the statistical information of the changing interference patterns. This article explains all the theory and some applications Heterodyne Near Field Speckles: from laser light to X-rays, , Advances in Physics: X, 6:1, 1891001 DOI: 10.1080/23746149.2021.1891001

[@mihirnakar4513]

I did not understand much but I’m still excited for it!! ahahaha

[@ronin6158]

Pretty impressive stuff. What's your opinion on light being purely a wave?

[@PhysicsExplainedVideos]

Excellent video! You managed to weave together a huge amount of content in under 30 minutes, without skimping on detail. I learnt a lot. Thank you

[@HuygensOptics]

Thank you for this compliment. It coming from you really means a lot to me!

[@michiel1512]

I'm so glad I found this channel. This is the real sience, with experiments, reason, and doubt!

[@sean_vikoren]

Agreed! Contrary to popular belief, real free-thinking scientists are rare.

[@harriehausenman8623]

@@sean_vikoren To be fair, that is mostly because scientists also still have to eat 😆

[@sean_vikoren]

@@harriehausenman8623 Agreed. As soon as possible, all scientists get free 'all the stuff (house, car, food, fun, no maid)' plus some kind of gamified equipment budget.

[@harriehausenman8623]

@@sean_vikoren 😁

[@vegandiver]

I love how all these experiments on your channel are things I’ve read about but never seen a demo of. In complete honesty I woke up at 3 in the morning when you uploaded this to start watching it. No other channel on KZbin captivates me like this one! Kudos!

[@HuygensOptics]

My notifications wake you up?? Wow, now that is dedication!

[@harriehausenman8623]

So true. Same here. (except for the waking up part ☺)

[@paint4pain]

The demo really helped me make sense of it, the hard part for me is understanding why Stimulated is temporally more coherent than Spontaneous. They both have the same wavelength and amplitude (after the filter) but since the stimulated emission is more orderly it's less quantized?

[@graealex]

A photon NOT being light is the thing that makes the most sense here. In the same way that an electron isn't electricity. Btw. you are on your way to 100k subs. Congratulations!

[@HuygensOptics]

Yes, I remember discussing this with you over a year ago when I still only had like a few thousand subscribers and thought it unlikely I would ever go over 10K. But in retrospect, making videos back then was just as much fun as it is now, so fortunately really nothing much has changed (apart from the time it takes to answer all those comments...).

[@Hunter271828]

This is a great video! It reminds me of an article by William Beaty called Lasers: What is Coherent Light? His point is you can make any light source spatially coherent by putting it in front of a pinhole, but it also makes it dim. He then goes on to say "And finally I know why lasers are so wonderful: lasers are pinhole light sources which are ...actually bright!"

[@RichardKinch]

That raises the problematic truth that lasers are not different in kind from other light sources. Photons carry no memory of how they were generated. Laser radiation is not necessarily coherent, temporally or spatially. Conversely, coherent light does not only come from lasers. So there is literally nothing special about laser light. "Nothing special", as in, not a distinct species. The whole world, including physicists, thinks of laser light as different from all others, when it isn't.

[@Hunter271828]

@@RichardKinch One question I have, talking about temporal coherence. Is it fair to say that having high temporal coherence is equivalent to the source having a very narrow frequency bandwidth? Like if you put a narrowband source in front of a pinhole does it have both high temporal and spacial coherence? I don't know of many narroband sources other than lasers though. Maybe like a gas discharge tube?

[@RichardKinch]

@@Hunter271828 Narrowband, yes. And potentially a very stable center to the narrow. A narrowband filter on a broadband source is necessarily inefficient (i.e., dim) and the inverse method of a narrowband laser source.

[@StefanHoffmann84]

I always wondered when people presented the double slit experiment due to Young by using lasers, often arguing that using lasers gives them the type of coherent light they need to see the fringes. But then how does Mr Young did it without lasers. Then once I saw a video by Veritasium where he researched how Mr Young did it back in the time. Basically, he made two small slits in a big black box to produce coherence. In the video this box is built btw.

[@BloobleBonker]

Can't wait for part 2. I've been scratching my head for years about these issues.

[@enotdetcelfer]

Your industry experience and the long time cooking your understanding is invaluable to understanding this beyond cliche textbook examples; thank you so much for doing these experiments for us and showing how to better think about these phenomena with cases beyond the flashy counterintuitive situations.

[@petergoose8164]

Very coherent presentation. The work you put in is amazing.

[@willo7734]

If I understand one of your main points correctly, it’s not light that behaves as a particle but the energy. Light itself is a wave but the energy involved (when transferred into or out of something else) behaves in a “particle-like” manner. That’s mind blowing and makes more sense than any other description of wave-particle duality I’ve ever heard. thank you.

[@jaydenwilson9522]

EM Radiation are open loops... while Chemical matter is closed loops... when radiation is absorbed into chemical matter is momentarily acts like a particle (closed loop).

[@uwezimmermann5427]

I teach my students that light is neither a particle nor a wave. Both are mathematically models which we can use to describe certain experimental observations. In my understanding, neither of the two (plus plain geometric optics as a third) models makes the claim to represent the true nature of light. We cannot in a better way (yet?) tell, what light is. We use the model which is easiest to use in order to explain an observation - I work with solar cells. The absorption of light in a semiconductor with a given band gap energy is easily explained with the photon model. The refraction of incident light through multiple layers with different diffractive indices, as well as the exponential Lambert-Beer absorption, standing wave phenomena etc are easily described by the wave model.

[@liex7187]

Temporal Coherence is really not the easy topic to understand it correctly. Thank you for such a great lesson!

[@mrcpu9999]

I have struggled to wrap my brain around this optical stuff for years, and finally just "Not for me". This was super eye-opening, and it all started to click. This channel is rapidly becoming one of my favorites.

[@jakesaari7652]

Fantastic explanation! Most other videos on the topic leave you with a sense of confusion as the presenter describes a magical process that defies common sense.

[@douginorlando6260]

Probably because they never understood it to begin with. The Physics Explained video he referred to is what should be taught to cut off all the confusion on the subject.

[@realcygnus]

Superb content as always ! This reminded me of a Feynman QED lecture where a guy in the class kept insisting that we simply "have to" use wave mechanics & Feynman was like, "NO Sir ! See, you already know too much". Calculating the probability of an event like a photomultiplier activating under specific conditions must be quite a different animal I suppose. At least that's my current nonsensical novice take. Anyway, I find ALL of this stuff very interesting. Such nifty equipment too. Really looking FWD to the rest of the series !

[@alexpyattaev]

QED falls apart hard once you start asking questions about electrostatic interaction. With all due respect to Feynman, his logic works well only for cases where you already have emitted photons. For static fields it does not apply very well at all.

[@ulwur]

So intrigued, can't wait for the second part!

[@horsemcgraw9631]

Always very excited to see a new video on your channel. Great work as usual, hitting the sweet spot between informative and entertaining.

[@aipsong]

Excellent video in all aspects - the clarity of the explanations, the pertinence of the illustrations, and the audacity to tackle such a difficult theme - many, many thanks!!!

[@caleballen4721]

I really appreciate your approach to answering your questions with experiment, and to be satisfied only when it makes sense intuitively, rather than being satisfied with a confusing answer from the consensus of popular ideas

[@Grateful.For.Everything]

🙏 how are you so damn good at making these videos for us! I hope everyone grasp the deeper meanings imbedded in your work, You really are a special one!

[@niteman555]

Your videos always show me how much more there is to learn about light.

[@ebrewste]

There are so many great channels on KZbin. Much better than any lecture I had in college. But yours is the only one that has me rewinding, watching again, thinking. Thank you!

[@BluesDoctor]

Timely as I work to explain spatial coherence to a client with a production optical problem. Thank-you for your insight and high quality experimental set-ups. You have given me more comprehensible arguments to particularly complex ideas. I hope I can return the favor one day in some small way.

[@LouisEguchiWale]

The most intuitive explanation of the most fundamental principles that are elusive everywhere else

[@schmetterling4477]

Also completely wrong. ;-)

[@fire17102]

18:40 is one of the best stapshots I've ever came across in my life. My intuition always told me that it's waves all the way down. Thank you ❤

[@alexpyattaev]

can't wait for part 2. The more I study about optics the more confused I become, and calling photons particles has never helped.

[@BRUXXUS]

Now this is incredibly fascinating. I love when someone can clearly present a new way of understanding concept. This one sets my imagination on overdrive. Thanks!

[@michaelkaliski7651]

Great work! Finally a practical demonstration that shows how light can appear as a continuous wave or discrete packets or photons.

[@markusreichel3896]

I have never seen such beautiful and well explaining coherence experiments during all my time at the university. Hats off!

[@executive]

this is really eye opening stuff. It really crystallized the energy/wavelength/uncertainty concept for me

[@Kavukamari]

this is becoming one of my favorite channels on youtube. I think these explorations will be extremely important in the future

[@LesLaboratory]

Excellent as always. I love your explanations, and find the wave model of light to be far more intuitive than the corpuscular model for most practical purposes.

[@HuygensOptics]

Thanks Les, I guess in lasers, there is no way around the wave character and it is actually quite difficult to find particle-like behavior. You can observe it in a detector when you attenuate a laser to very low intensity levels, however that has nothing to do with the light consisting of packets of energy. It's just the response of the detector to temporal constructive coherence.

[@BrunoMe]

Both of your videos are fascinating. The graphs at 18:40 and Les' most recent videos give me some idea as to what part 2 might be about.

[@johnsanford8865]

I am a huge fan of these Huygensoptics videos. I use these with my students at UCSD. Thanks for the great work!

[@daniellewilson8527]

Thank you for explaining temporal and spatial coherence. There was a different video I watched that mentioned them but didn’t explain either RC or SC very well. I’m glad KZbin recommended your channel

[@martin09091989]

Your videos are such a joy! Why can´t every human be like that, and share there knowledge?! Specially if it is that fundamental. 🙏

[@hexane360]

Great video! I agree popular descriptions of QM get a lot wrong. I also think you're correct that Einstein was wrong in saying energy must be localized in a photon. I think a lot of the confusion re: wave particle duality comes from presenters (understandably) trying to skip teaching the basics of quantum mechanics before jumping into applications. It's very difficult to understand what physicists mean by terms like "superposition" and "uncertainty", and very easy to substitute in your own intuitive definitions. With that disclaimer, here's my shot at a high level explanation: I prefer to think of "wave" and "particle" as two "perspectives" you can view the same system from. If you make a wave-like measurement of the system (e.g. frequency), you'll see a wave. And if you make a particle-like measurement (e.g. position), you'll see a particle. Quantum mechanics says that both of these perspectives are valid, and furthermore: If you "project" the system into one perspective, its state from the other perspective becomes indeterminate. This means that if you make a wave-like measurement, it's impossible to determine the particle it originated from. Further, there's no one "fundamental" perspective: you can consider a particle as a wave packet consisting of a sum of many waves, and you can consider a wave as a sum of many particles. The math might be easier for one, but there's nothing special about the wave formulation. Everything I said above applies to *all* particles, not just photons. Most people are more comfortable thinking of electrons as particles, but their wave-like behavior is incredibly important to fields like solid-state physics. Another good comparison is the phonon, which is widely accepted to have particle-like behavior despite existing in a field. When you get even deeper, even the "more substantial" particles are quantized packets of their associated fields. So, philosophically, whatever you call a photon, you should call the electron, phonon, and proton the same.

[@EpsilonZRho]

Thought provoking and educational as always, Jeroen! I'm looking forwards to where you're going with this. I implore you to consider into your thinking the typically neglected process of EM generation by means other than orbital electrons transitioning to lower energy states: EM radiation at microwave frequencies and below, basically RF. At those frequencies, "photon" generation can't be explained in terms of orbital electron transitions afaik, and so I think by understanding the factors common to both types of EM generation, we can better grasp what a "photon" is (and isn't).

[@HuygensOptics]

I hope I have time to incorporate the equivalence of micro- and radiowaves to visible light in the second video. If not, I'll try to make a third video. Contrary to what some want to make you believe, there is not fundamental difference between the nature of the radiation. The difference in behavior is just due to wavelength and number of discrete radiative emitters.

[@EpsilonZRho]

@@HuygensOptics I certainly hope the topic makes it into one of your future videos! The point I was attempting to make isn't that there's a fundamental difference between the various categories of EM radiation or how it's generated, but rather that the common treatment given for photon production provides pretty limited insight into the actual physics that produces these waves/photons. For example, it seems to me that an electron falling to a lower energy state explains when/why a photon is released, but not how.

[@Leomurguia]

I really appreciate the quality of your explanations. Thanks

[@filipjanko4334]

I was waiting for this kind of explanation for years.. here I have it. THANK YOU

[@MissNorington]

Awesome video for my birthday! To explain why: This video actually beings up all those other videos I have watched that didn't make any sense, and actually went on to more interesting things with light than click bait science.

[@HuygensOptics]

Happy birthday! I guess this is my present for you then ;-)

[@MissNorington]

@@HuygensOptics Looking forward to part 2! :D

[@harriehausenman8623]

Yeah! Happy Birthday! 🎂🥳🎆

[@AK-vx4dy]

Amazing how you crush all my previous misconceptions and i am very glad of that fact

[@harriehausenman8623]

With you videos, I always feel like we are pondering the questions together 🤗

[@georgH]

I saw the title and thought I would watch it later... Until I realized it was from Huygens Optics and clicked immediately. I really really enjoyed your videos of the mini telescope!

[@gauravkumar-vj2qt]

The channel remains loyal to its name. Loved the video.

[@mikeymcginley]

Hello Jeroen, thank you for all of the great content you have documented over the years. I find myself having to rewatch your videos several times over before the concept at hand starts to make sense to me - all the while enjoying the tingling sensation it brings to my brain. Could you recommend any reading material that could help me catch up on the fundamentals behind your experiments?

[@zerden8303]

Its great to be able to watch wave phenomena introduced with such a clarity :)

[@benmcreynolds8581]

I love this channel so much and I'm so glad I've subscribed to it. It's such good content. Like food for your mind.

[@hedgehog3180]

As I learned more about physics it slowly dawned on me that the weird thing about wave-partical duality isn't reality the duality but that light would ever behave as a particle at all since the wave model works so well. I think however that the particle model became so popular because it's incredibly easy to explain and it works really well as an explanation in most circumstances. Like the particle model works just fine for chemistry, biology and engineering and it's much easier to conceptualize than waves. Quantum mysticism shows us that people have a hard time understanding waves at all if they don't have any science education so it's not a shock that pop science generally relies on the particle model.

[@OrenTirosh]

Ever since I first heard about the so-called wave/particle duality many years ago I had the intuition that this had to be the case. That the electromagnetic field is “really” a wave and just the interactions are quantized into discrete packets of energy. Nice to see it explained so beautifully. Can’t wait for the next part!

[@Whina_Bell]

This has been so helpful (and super interesting!)Thank you!

[@optiondrone5468]

Simply put a masterful explanation, thank you for sharing this!

[@Surukai]

Mind blown, and I've worked 10 years with VIS and NIR spectroscopy but you just gave me so much more understanding I wish I knew earlier :)

[@JustaReadingguy]

I had thought about some of these issues for several years and you gave me some great answers. Wow thanks.

[@tolkienfan1972]

I'm always excited to see a new video from Huygens! ❤️

[@dougthomson5544]

Thank you for pointing me to this video. I just found your channel recently and I enjoy your teaching very much.

[@ssssssssssss885]

Wow! The most compelling video on this subject ever shown on yt. Most creators use graphics only. You, sir, build full lab grade experiments. Thanks!

[@ptonpc]

Finally getting the peace to watch this and the next video. Thank you.

[@sinephase]

Really cool video! I never considered before that if a particle travels the speed of light, the only way to have different energy states is to have it change frequency, so "slowing" light would increase its frequency and that explains how optics redirect light! Neato! :D

[@phy6geniuxYTcreations]

Wow, this helped me understand my studies on metamaterials. I know this is a part of the fundamental physics of light but presenting the concepts and experiment like this really motivates the audience to think further. Thank you!

[@richardcoppin5332]

This channel just makes things so real.

[@lukemurray-smith5454]

Thanks for this video its amazing, just subscribed. Also is there anyway to think of or research done on objects with enough mass or gravity to curve space so that the observable universe relative to them might actually be larger? A black hole for instance might be able to "see/experience" beyond what the earth can observe due to the curves of space time keeping frequencies within a finite bandwidth allowing for greater interaction?

[@EngineerNick]

This was amazing thank you :D I love that you can show the practical experiment

[@arjenchristianhelder1027]

Dankjewel Huygens! heel mooi uitgelegd, en met alle metingen en visualisering is het goed te volgen en zeer interessant!

[@fredinit]

I feel that you have come closer to explaining the "true nature" of light than just about any explanation I've run across. Is it a particle? Is it a wave? It is both, neither, and more. Light is as it is - not as the theory says. Dual nature? Sure. Extends from the macro (electricity / audio frequencies) to micro scales (gamma rays/wavelengths), why not? Reminds me of a discussion about one of Apple's latest products - that had tiny laser drilled holes for the speaker. And folks (some professional physicists with lots of letters) where stating that it can't / shouldn't work. The holes were less than 1/2 lamba of the wavelength blah blah blah! They had spent so much time with their nose against the blackboard using sine waves, trig, and calculus, as a model for what sound was they forgot what sound is - the compression & refraction of the medium through which it travels. The holes are nothing more than screen, cloth, etc. So long as they are large enough for the medium to move, sound will too. Once we truly understand (I'll borrow the term 'Grok' to mean 'true understanding') what light is, I believe we have many many more advances beyond where we are at now. Thank you for helping to advance the experimental, theoretical, and philosophical nature of this topic.

[@HuygensOptics]

In the case of the tiny holes, they are so close together that they sound like a single source. Their sound emission is temporally coherent and at some distance, the sound will be spatially coherent as well.

[@rutger4131]

The first half of this video is very reminiscent of a paper by W.E. Lamb jr. called 'anti-photon' where he states that only a small number of people should be allowed a permit to use the term. The only case in which it makes sense is as a quantum of energy of a harmonic oscillator for the electromagnetic field. The energy of such an oscillator is E = (n+1/2) h f. The number n is a photon. An (attenuated) laser is (approximately) in a coherent state which is an infinite superposition of different n-values. That is why you can't speak of a single photon in an attenuated laser setup, as you demonstrated in your 'how big is a photon' video. You can only speak of the average number of photons and that could be 1, or something much smaller or bigger. This 'pseudo'-single photon source is good enough for some applications like quantum key distribution by the way. You may also like an article by Art Hobson called There are no particles, only fields.

[@Pidrittel]

Both the "no photon" paper as well as the "there are no particles" are very interesting, thanks for sharing!

[@stevehosier7378]

Excellent! I am really looking forward to part 2!

[@4n2earth22]

That dull yet snappy sound was my mind blown. To bits. (edit) Field Physics is the key to understanding discrete phenomenon.

[@ArrakisMusicOfficial]

Please, please, please, keep diving into these basic concepts and misunderstandings. I recently had a revelation that for most fundamental concepts, there still wasn't made the perfect video that will provide you with an intuitive understanding of a phenomenon or at least an intuitive understanding of your own previous misunderstanding. What you are doing here is exactly that, and coming much closer than anything about quantum physics on YT I found so far (and I've been passively looking and watching for the past 4-5 years), narrowing down the question; "Why are we still confused about wave-particle duality?" to the assumption that photons, as packets of light are real. My brain is already going full throttle redefining the way I construct thought experiments about EM fields, atomic particles and waves. What I got from this video is that photon is a word, that attributes particle properties to a phenomenon that requires nothing else then wave superposition. Thank you for the gaussian explanation, this finally answered the question that puzzled my mind for a long time, how do you get infinite, continuous waves to form travelling, seemingly discrete packets. It's this ever shifting phase that creates this phenomenon. What I still don't quite grasp is the temporal aspect of it. Since we're talking about waves in a field, there must be an event of "shaking the substrate" that generates those waves and then probably some elastic damping going on until the source goes quiet. It would seem, that if you shake the substrate just for a little bit, you would get this wave packet anyway, since you were not generating waves before or after the event. The result should be the same. How does this tie in to the idea of waves that extend infinitely in space and time, yet when added up, they present you with this wave packet phenomenon? Is it purely a matter of point of view? If so, then infinitely extending waves are just a useful abstraction. The second thing that left me puzzling (as intended, I'm sure) is the absorption event. What does all this mean for quantized absorption? If we assume, that the EM wave passing by the atom starts shaking it and at a certain frequency, the atom can with certain probability decide to immediately absorb all of the shaking and use it to bump it's energy state. The question is - as time passes, the energy of the wave inevitable spreads in space. If it's spread in space, how can it be all of a sudden localized again? Unless it's another emergent trick of wave phenomena. The non-intuitive thing is how can energy of a "photon" be dependent on its frequency? Intuitively, as the wave spreads out, it still contains the same frequencies, but with lower amplitude, there simply must be an amplitude component to the energy of the final wave. The energy could then spread out "continuously" with aplitude, as expected, which would then lead to the conclusion that the absorption event can happen even at very low amplitudes, though with lower probability, since it's not the amplitude but the frequency of the local field that matters. So the claim that "photon's energy depends on it's frequency" would then be very, very, very, very misleading, since it holds true in its convoluted context, but doesn't generalize to intuitions about the wave phenomena, where the amplitude is the main "energy" component. However, now thinking about acoustic waves, same holds there as well. You have air particles moving back and forth, following a sine wave, if you have pure frequency. Given amplitude 1, the particles move distance 2 over the period of one wavelength. Since you have moved a mass a distance, you can compute work and then for low frequencies, one wave cycle takes a long time for the particle to travel, whereas in a high frequency it will travel the distance many times. So inevitably more energy must have been in the system, since more work was done in the same amount of time. I suspect this will be somehow analogous to the behavior of EM waves. Maybe I'm just babbling non-sense, but these are truly questions puzzling my mind I'm trying to figure out :D

[@c2h5oh77]

It seems that according to the Compton effect, photons have directions as well. This place needs careful consideration

[@miklov]

Fascinating. Looking forward to the next video!

[@4pharaoh]

Just found your channel and subscribed. Brilliant work. Generating new questions... @ 18:41 clearly these patterns are the frequency content (spectrum) of a single square wave pulse. Is light (a photon) a pulse, and continuous light overlapping pulses cancelling out all wavelengths other than the fundamental? Looking forward to part 2.

[@artem_kurov]

Amazing story, looking forward to second part

[@TheWyrdSmythe]

Excellent! Looking forward to part two!

[@matiasfernandezlakatos5882]

Great explanation, congrats. I'm moving to video number 2 right now.

[@fotografm]

Thanks for the link to the Physics Explained channel Wave Packet video. Now I am wondering if you could make a video demonstrating the production of single photons. Simply reducing the intensity until there is only ony photon left never seemed like a very elegant solution to me. I love to ee you actually demonstrating things with real hardware !

[@sebaschtl9710]

Thank you so mutch. You explain so well. it is pure fun. I love to watch your videos.

[@SR-ml4dn]

Big Thanks to you Jeroen for this awesome good video. I appreciate all your videos but this one really stands out and try to explain some long hair content not too simplified. When looking at the wave packet it just remain me about the mathematic for Soliton equation and the behavior, that Soliton can travel through another wave or Soliton without getting distorted when it come out again, same as your two coherent light beams crossing each other. Also thanks for the music video I am a big fan.

[@KitagumaIgen]

Brilliant! Looking forward to watch the next video.

[@Xplorer304]

Thank you for a very illustrative video. I really grasped the concept of "pulse" and why it has the ability to excite a broad band of frequencies in NMR spectrometry. I've got a question. I stared to have a doubt if a lot of consumer pointers that are sold as a "laser" are actually lasers, and not just glorified collimated photodiodes. Is it so?

[@andyeverett1957]

What a privilege to be able to watch this video. Thought provoking, thank you.

[@andyeverett1957]

@@schmetterling4477 What part is false?

[@andyeverett1957]

@@schmetterling4477 The guy who made the video is a expert in the field of optics, give one example where his video was wrong. Anyway his video is not bullshit far from it, a great work of science. I would watch a video made by you, please make one as good as the above.

[@andyeverett1957]

@@schmetterling4477 "“These days, every Tom, Dick and Harry thinks he knows what a photon is, but he is wrong,” and he directly said to physicists that “Every physicist thinks that he knows what a photon is, I spent my life to find out what a photon is and I still don’t know it,” and sometimes before his death in 1955, Einstein wrote “All the fifty years of conscious brooding have brought me no closer to the answer to the question: what are light quanta? ........... Of course, today every rascal thinks he knows the answer, but he is deluding himself” (4)." The opinion of Einstein. No course on quantum optics would answer Einstein's question to his satisfaction in my opinion. He was looking for an idea much deeper that you probably would say does not exist and I would say you are wrong.

[@harriehausenman8623]

Absolutely wild ride again! 🤠 Thank you!

[@MisterItchy]

I didn't understand most of this but it was still interesting! Thank you.

[@TickyTack23]

15:30 My thought on this, which be kind I don't know much about physics other than practical applications. When a photon is detected/absorbed, it's really seeing the influx of energy from the waveform collapsing onto that photon point. Am I thinking correctly? Or way off?

[@cylosgarage]

Dude! Are you reading my mind?? Just as I’m trying to learn more about coherence the PREMIER optics teacher on KZbin posts a video on it!! So hyped to watch this

[@HuygensOptics]

I do read minds occasionally, but always within the boundaries of statistical probability.

[@harriehausenman8623]

@@HuygensOptics I only read minds of Boltzmann-Brains 😆

[@FredStam]

Dank voor deze uitleg van een moeilijk onderwerp. mooie presentatie met fraaie animaties. (Thanks for explaning such a difficult subject. nice presentation en beautiful animations)

[@carlbrenninkmeijer8925]

Fascinating, so clear, many thanks !!

[@weirdsciencetv4999]

This is amazingly well done

[@ChristofVorhauer]

Thank you, very well explained, I've learned a lot, waiting for the next video

[@keiferstewart5010]

Absolutely love your videos! Keep it up

[@glentyan2505]

Absolutely marvellous presentation {again} and graphics, genius is a word I don't use lightly but feel compelled to use for the author of this channel.

[@brandonhicks7549]

It’s interesting to see the take of this subject with an optical perspective. I come from the land of microwaves and radio frequencies, where Doppler spectra and delay spread limit the coherence time and subsequently distance. I look forward to your demonstrations on spatial correlations, which we usually talk about in terms of angular spread. Knowing a bit about the gaussian beam as a valid solution to maxwells equations, I expect the results will be interesting and explain a lot about the difficulty in fast optics

[@jandenbrok9574]

WONDERFULLY clear!

[@junkerzn7312]

This is why I love this channel (!)

[@UliTroyo]

This was fun and informative. Thanks so much!

[@user-ft5qk4nv4f]

Would it be correct to say that a photon is the virtual particle moving energy from the electron in the shell to the electromagnetic field and vice versa?

[@turun_ambartanen]

Amazing video. I'm not up to snuff on my laser physics, but the results shown at the end would mean that for stimulated emission we have a much more precisely defined band gap (or whatever is between the excited state and the base state) in the material. Whereas with spontaneous emission we have a much wider range of energy gaps across which the electrons relax. Using the equation shown at 19:15 I get a DeltaNu of 90meV. That's not a lot, but much more than I would have expected. 635nm means a 1.95eV wide "default" band gap, so DeltaNu is 1/20th of that. I wonder where the difference comes from. I know that an exciton (electron/hole pair) in a semiconductor creates its own potential well, but that would only explain deviations in one direction, a lower energy gap, so it can't be the complete answer.

[@HuygensOptics]

I guess there is a lot of things that can contribute to spectral broadening in a doped lattice, like statistical variations in the surrounding of the emitter. Also, a variety of thermal effects should be taken into consideration, which cause exited state life time variations. And- contrary to my expectations- the transition to lasing was actually way more gradual than I expected. Anyway, this is what I measured.

[@turun_ambartanen]

​@@HuygensOptics I don't see an obvious connection from the life time of the excited state to the energy of it. The statistical variations certainly contribute to the spread. I don't know enough to quantify their influence. EDIT: So I did some research and feel kinda stupid for not thinking about it earlier... A Laser diode is just a regular pn-diode, in a semiconductor with a direct band gap, with a current sent across the depletion region in flow direction. Until we reach the threshold voltage, aka before we begin lasing, we have only rare charges traveling across the depletion region. Those charges can either recombine across the bandgap, but they can also tunnel a bit, trading the tunneled distance for a difference in energy difference (because of the internal electric field of the depletion region, distance equals energy for charged particles). The probability of the resulting energy gap is not trivial, but I can easily imagine that this results in a gaussian distribution. 90meV are also easily explained with this. Depending on the doping concentration an electric field with 1V/50nm is easily possible (by intuition, didn't do the math), meaning 5nm would have to be tunneled which is realistic.

[@douginorlando6260]

A good visualization of Schrodinger’s time dependent wave equation is the 4 minute video in the “Above Threshold” channel called “A quantum mechanics simulation with an explanation”. Although it’s a calculation of the dual slit experiment for an electron, it is very similar to the lab measurement results obtained by Huygens optics in his dual slit experiment. It even includes standing waves inside the dual slits themselves.

[@YodaWhat]

Excellent episode, from an excellent channel!