How lasers work (in theory)

How does a laser really work? It's Bose - Einstein statistics! (photons are bosons)
Check out Smarter Every Day's video showing lasers in real life: bit.ly/uBwhU2
Trying out a new feature: English Transcript! Let me know how it works
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Minute Physics provides an energetic and entertaining view of old and new problems in physics -- all in a minute!
Music by Nathaniel Schroeder
youtube: bit.ly/pakJLE
myspace: mysp.ac/qtmZQj Created by Henry Reich

[@BinkieMcFartnuggets]

I tried explaining this to my cat and he's still baffled.

[@guymcguyenson3858]

Bob O. wtf? >.> i done runned over yer cat last munth, hes not baffled no more. shuck-a-muk!

[@raktoss1474]

Bob O. Mindblown.

[@guymcguyenson3858]

my cat "gets" it, i don't see the problem, try harder

[@mrphwibbs9766]

My cat is behind this whole science show.

[@Blade.5786]

@@guymcguyenson3858 It's been 4 years,so it's highly likely your cat's daddy is dead.

[@TheErich2468]

Photon peer pressure :(

[@MrNecroDrew]

good one!

[@Silverwind87]

Erich Shan All the cool subatomic particles are doing weed?

[@realbignoob1886]

Lmfao

[@teamofreally]

I found love the last time I got trapped between two mirrors too

[@dimitrispapadakis2122]

that's sad

[@konnorporter5115]

Do you mean a mirror maze???

[@Ajayiikponmwosa]

You're loosing concentration.

[@jahmed5750]

❤️❤️

[@backyard282]

This video explained lasers in such an incredibly immensely simplified way that no one can really be satisfied with it, or, let alone understanding the concept.

[@fabiena1787]

So true. Explaining lasers isn't simple. But this isn't even the beginning of an explanation. This is the kind of false explanation that you will find interesting if you already know how it works.

[@FunkyFlutist]

@@fabiena1787 what’s the proper explanation? I think it was pretty good for a 2 min video.

[@kieranhosty]

I've found this explanation here, currently watching fully to see if it appropriately answers the question, but it should be a good starting point for other questions you may have. kzbin.info/www/bejne/enKakpKsbpt9qdk

[@anneashley5110]

You know, these physicists delight in complicating matters so don't blame your cat. Just pass a Ray of light through a man made quartz crystal which does have the power to disperse that light so it comes out pint point accurate according to what beam intensity you put in.

[@GarenPhillips]

So are photons in their own field surrounding us at all time but we only see the ones that phase in?

[@KevinAnimalCrossing]

1:06 best love story ever. ;(

[@drorbazer7531]

Kevin AnimalCrossing Still a better love story than Twilight

[@Chris349]

Dror Bazer which is still a better love story than 50 shades of grey

[@sulaimaanchamda4501]

i dunno its kinda creepy 1:08

[@Smeiksmeiksmeik]

in only 2 seconds it turns from aww to pedo

[@mon0522]

kevinalb007 Better love story than twilight

[@tenaciousdean6179]

So THIS is the first step to making a lightsaber!

[@bomblii]

This is the power of friendship

[@JamesFromAustralia]

If he could explain why these things happen in a KZbin video people wouldn't need to spend years at universities. These videos are for entertainment and to inspire people to seek their own answers, so thank you MinutePhysics for making up these metaphors, it's things like your videos that are the reason that I'm studying Physics today.

[@eddie246]

1:16 oof look how happy those three look together awie 💕

[@MilanKarakas]

So, how the LASER really works?

[@oxcart4004]

This video explains the one thing I had not been able to find, and that is... "how can the photons be in phase if they are not generated at the same time." Also the coin flip analogy really helps understand the process. There are so many great articles out there that explain how lasers are built and the basics of what they do, but without the points from this video you can't really get an intuitive understanding... Thank you for the great overview!!

[@Raznak]

I did not get the coin flip analogy

[@trollkingb2000]

clicks on one vid of minute physics *five min later* i cant stop watching vid after vid

[@troyhayden3773]

I love these! I'm sure my physics teacher would be agree

[@oadka]

Nice! And very concise!Thank you!

[@AcheterlaserVideos]

Excellente vidéo explicative ! Superbe réalisation

[@paolotenis20]

You guys are awesome! Helped me to understand the lasers for my physics research. Thank you!

[@bsaget98]

149 photons were different

[@princesoni224]

The best explanation I have ever seen! Great

[@didaloca]

I always love the bass jammin' away in the background in these videos.

[@ricardo.mazeto]

The best explanation I have ever seen! Great!

[@morissmor]

The better grammer i have ever seen

[@ricardo.mazeto]

+Reptile estrin LOL.

[@AndrewGlitchMasterBalaschak]

I have actually seriously thought about this sometimes, sometimes, except with light instead of lasers, with light I am pretty sure it would be un-observable because even the smallest crack would allow light to escape, and because light moves at the speed of light and the light source is no longer there we will never know if the light dims in the room of mirrors. Is this correct?

[@baharosman1416]

After watching to many videos , this video finally made me to understand

[@UniverseIsAwesome]

Awesome :D :D :D. Thank you soooo much for your amazing explanations on scientific topics!!!

[@sanwalfarooque2747]

next thing you know .. people are fighting for photon marriage rights ...

[@Oryon44]

Please don't give ideas to Tumblr

[@nispelsm]

Star Trek Voyager did it.

[@AffeAffelinTV]

yeah because marriage rights for gays is such a rediculous idea that you have to parody it

[@CanyonF]

*#howtotriggeratumblruser101*

[@yuvalsela4482]

@@Angel_foxxo its not unatural. We see it in the animal kingdom.

[@podgemcgrath]

It's pretty amazing to think that the light that comes out of a laser is only 4% of the light created within the laser.

[@joshuagross6022]

Henry, apart from thinking your videos are fantastic, I LOVE your choice of music.

[@kfqfguoqf]

Cool concept, the one with the fotons. Your analogy was good too.

[@AttilaAsztalos]

Wow, now I know what Einstein meant by "everything should be made as simple as possible, but no simpler"...

[@alexanderx33]

"Photon's want to be together", I feel like we are regressing a bit here. A little more detail please, maybe refer us to a sixty symbols video at least.

[@fecalophagus]

Thank you very much for sharing information!

[@TheBackwardBros]

I love lots of your videos, but this one was extremely simplified i coullndt learn much in this video D:

[@CalebDiT]

Explaining what photons do is not the same as explaining why they do what they do. For example, in showing that there is a greater probability of finding photons in the same "state," (at 00:50) you still don't yet have any rationale for giving a "Therefore...." This probability has nothing to do with why photons from the output of a good laser are in phase. Laser designers don't say, "The light will probably be in phase." Certainly you'd agree that their design specifications have something to do with it. If the specifications are more relevant than probabilities, why not include them in a "How lasers work" video if the intention is to show how lasers work? It may be worth your while, also, in studying why the theoretical physicists some years ago said laser light wasn't possible, and how they treated the people who said it was. Some of the speculations in modern physics have to be let go, it seems to me, considering the fact that laser light is obviously possible.

[@thomasmaxfield8953]

Caleb I totally agree, but the channel name is minutephysics. I think he did alright with the brevity but he could have alluded more to some follow up for more interested viewers.

[@KatieKat223]

ALL YOU HAVE TO DO. /ALL/ YOU HAVE TO DO.

[@VamanNeurekar]

Please introduce yourself, the person who makes these awesome videos...

[@susanxjx]

Thanks for explaining!

[@katiebruce6164]

if you just slowed down talking a bit..that would be really helpful.

[@BoBoN4Uto]

KZbin have 0.5 and 0.25 video speed

[@AtoHenok]

"The most romantic love story I have ever heard" - Sheldon Cooper

[@iSuperdupaloveweed]

this is really awesome. this explains why if you look at the reflected dot from a laser on certain surfaces, you see the much smaller individual dots making it up moving all over the place within the dot

[@sayaelnu]

Your videos make me think of science in a very adorable way.

[@dg-hughes]

LASER (Light Amplification by Stimulated Emission of Radiation) is an acronym so it isn't a Laser it's LASER beam (of light), as said in the video but I don't think people understand that subtle point.

[@INVISandMOONLIGHT]

+David Hughes Well, it's more commonly referred to as simple 'a laser' or general laser light, regardless of any formalities or technicalities. So as long as everyone understand what it is, eh?

[@TheMilkManCow]

+David Hughes It's become just a word in the English language just like Kleenex is interchangeable with tissue

[@Dannys99887]

This is a pretty poor description. How can you meaningfully discuss the operation of a laser without the concepts of phase coherence and population inversion?

[@Chaossdruid]

He isn't 'meaningfully discussing' the operation of a laser; he is vulgarising the process in order to make it comprehensible and appealing to a larger audience whilst keeping the explanation concise.

[@isodoublet]

Chaossdruid He isn't "vulgarizing" it, he's utterly raping it with a chainsaw. This explanation was almost completely wrong. It always baffles me how people think that an explanation being "accessible to a larger audience" makes it any less wrong. Uh, hello? If it's wrong, it should *not* be taught to a larger audience! Leave the wrong, appealing bullshit to scifi writers. Science this ain't.

[@General12th]

gogerychwyrndrobwll There's a common analogy teachers use when teaching basic arithmetic. Imagine a man walking forward on a train that's also moving forward. What is the man's speed relative to the ground? According to the teacher, it's the man's speed on the train plus the train's speed on the ground. But that's false. Relativity tells us his speed is actually a little slower than that. By your logic, teachers shouldn't use this analogy because it's factually incorrect. Their students would go into the world with the wrong idea about how the world works. It wouldn't matter that the students' understanding and the reality is off by mere trillionths of a m/s: wrong is wrong, period. Maybe you're actually right. This video isn't accurate, and it's off from the reality by a lot more than the train situation is from reality. But unlike basic arithmetic, which has many possible analogies that a teacher could use, the physics of lasers is awfully complicated. This video shows what very well may be the simplest analogy possible that gets the basic idea of lasers across. It might be technically wrong, but a basic idea is better than no idea at all, right? If you can think of a more accurate but equally simple analogy, I would honestly love to know it. I want to know more about lasers, too. You could even try making a video about it. If it's as entertaining and even more educational than these videos are, people will love to watch it.

[@isodoublet]

*****"By your logic, teachers shouldn't use this analogy because it's factually incorrect. " That's not my logic at all. That's your supposition of what my logic is. The explanation given by the teacher is the low energy (Newtonian) approximation to the full relativistic dynamics. It is the leading term in an asymptotic expansion in v/c. It's clearly appropriate to the problem at hand. Further, it's *more* appropriate than the full relativistic expression since any and all corrections that are of higher order in v/c get entirely swamped by experimental error and are unobservable, even in principle. Strictly speaking they can't be said to be there at all. So the Newtonian approximation is entirely justified, rigorous, and accurate. In contrast, I know of a certain physics youtuber who dislikes Newtonian approximations... watch?v=IM630Z8lho8 D'oh! The problem with the exposition in this video is not that it's "simplified", or even "outdated". It's *fictional*. It was never true, it never even resembled anything that could be construed as an explanation for a laser. You can easily see that this simple entropic argument cannot work by noting that if it were true you'd see lasing merely by placing on mirror in front of the other. This obviously doesn't happen. You *need* to understand stimulated emission and gain media. You *need* to understand population inversion. Population inversion is quite interesting by itself, since it leads to the subject of negative temperatures which are surprising and fascinating. A competent physics youtuber attempting to explain these concepts would have to find creative ways to make them accessible. Veritasium is typically very good at this. Minutephysics, on the other hand, tends to just make up whatever fictional explanation would sound sexy, even if it has only a passing resemblance to reality. "You could even try making a video about it." I've been meaning to try to start a physics channel, but honestly I don't think I have the competence or charisma to make videos that the average person would find interesting.

[@General12th]

gogerychwyrndrobwll "D'oh"? Nah, I used the man-on-the-train situation for that very reason. Glad you caught that! So, you're right. Henry's reasoning doesn't approximate reality in the slightest. (Side note: From my own education, he's usually not wrong in most of his videos. This video seems to be an outlier than an accurate trend in Henry's scientific accuracy.) But that raises a question I've seen other people approach in this video and others: namely, is it possible for his more... interesting ideas to be wrong in principle but right in practice? Henry's videos may be factually incorrect, but they do expose a broad audience to interesting scientific phenomena. So, is it better that people watch these videos even if they're wrong? Or would it be better if people didn't watch these at all? I do think the former is preferable, since intrigued viewers might go on to learn more about the subject on their own, but that's just an opinion. I am curious to read your own, if you care to share. I've been meaning to start my own physics channel, too, but I haven't found the time to write a script or set up a film area. Also, yeah, making videos interesting would be another problem to handle. Still love to try though.

[@MegaTonyHawks]

Thanks For Translate.Köszönöm a fordítást.

[@INVISandMOONLIGHT]

Cheers mate, really great when you have a video on a topic for my chem project =P Making everything a lot easier

[@InsomniaS3S]

Yes! I only just discovered them like 3 days ago and have been watching non-stop.

[@dimpiroy8992]

superb!!!!!!!!!! such a nice explaination

[@silencedidgood]

I LOVE THE BASS LINE!

[@ohnogrl]

i cannot believe i am 23 and saying this, but the background music sounds like when steve is drawing a clue on blue's clues. on another note, i love this channel. i learn so much...(after watching the same video like 3 times in a row)

[@livin4thelamb499]

this is the happiest way to explain lasers ever!!!

[@Snailien_loves_you]

This is the only one of your videos that makes sense

[@Deadmau5wannabe]

this really helped me for my science project

[@janelle6734]

Thanks for explaining this to me 😁

[@magnuslaursen9203]

Awesome video :)

[@simoncooper2663]

I knew it was concentrated light but this explained it much better than I could have.

[@saravana_1999]

Literally explained it in the most coolest way possible and understood it the first time itself

[@jebuzjaime]

You my good sir deserve an award.

[@herman8rrr]

glad to hear it

[@EddieTheBunny]

Ah the science section of youtube. Sometimes it even seems like you're on a completely different website because it's amazing. :)

[@Hajzelnut]

Very nice said. :)

[@ankurmandal4792]

Back in the days when minutephysics was minute physics

[@kaioborges6432]

Congratulations form Brazil extremely good

[@Mothman453]

Cant..stop..WATCHING THESE!! >3

[@KennethHopkins]

really cool double bass

[@Jamilton73516]

Funny... I looked all over the web for a simple explanation... I actually noticed this and went on (0n my search). After a while, I got tired and watched this! It solved all my problems :3

[@adrienmau8082]

Hello ! I've seen many of your videos and you are doing a great job :) , I'm currently bulding an app to teach young people about light and optic, and I wonder... would it be possible to use this video about laser? This video is both funny and pedagogic, and that's exactly what i was looking for. The app will be free, of course. Thank you :)

[@lonelyPorterCH]

cool, now I finally know, how it works^^

[@vvvcodevvv]

Thank you so much

[@ZaMarch16]

I love how whenever people try to explain the universe simply they're like "Well imagine that time is a line." then later on it's all "Time's not a line, that was just an easy way of explaining it."

[@viktoriahart3627]

I

[@AbelTaylor]

you can, if you use the .9 infinitesimal rule. also, there are systems for counting besides the metric system. if you use the dozenal system it is quite easy to express two thirds, and 1/3 as decimal.

[@MIROMARMORA4]

why everyone is hating , he's trying to simplify the idea so that everyone understands and i think you did a great job .. I take physics this year and the video explained a lot of what i didnt understand .Thank you

[@vaichild]

i wish i could give my high school projects like he does his videos

[@ronakknikam]

If you have prior knowledge of this stuff then this video will be a best summary

[@RoadkillIndustries]

damn you vsauce! now im addicted to yet another channel that has really smart stuff in it and makes me look smart to my freinds!

[@Theeemadd]

nice way to teach...

[@brendanguansing9227]

I love to be part of the minute physics community☺😊

[@Deathbysnakes90]

Go fuck yourself.

[@Drostie]

It's a little more special because you can do it with integer-spin particles (like light, sound, gravitational waves, and the charge-pairs in a superconductor) but not half-integer spin particles (like electrons, protons, neutrons, neutrinos). Light is indistinguishable in a way that it can "condense" into the same state while electrons are indistinguishible in a way that only one can occupy a state. Once you have that sort of indistinguishability, though, the probability does work that way.

[@TheReaverOfDarkness]

Common cheap lasers use a rod made of ruby (corundum) as a medium, with an electromagnetic coil wrapped around the rod. There's a lot more to it, but somehow energizing the coil causes the ruby rod to generate a red light in a focused beam. I think they might also use a lens to focus it even better.

[@OviMG]

amazing

[@ikemkrueger]

I like your drawings. :)

[@DoodleAds]

Excited by this video - or should I say in a higher energy state LOL

[@icedragonair]

I love stuff like this because it basically illustrates using maths to break our understanding of the universe. When i first learned about the uncertainty principle, i parsed it as "we can only know position or velocity because of the way we measure" implying that there was a way to measure both, even if only in magical theory land and never in reality. Because we tend to relate everything to what we can experience, i was still subconsciously thinking of electrons and photons as "objects". But i later realized that the duality is litteraly unknowable, even theoretically, because photons and electrons arent like anything were used to. They arent lines or dots, and they dont operate on the kind of logic we do. I once was watching a lecture on how one could sorta try to visualize 4D shapes, and it just became so obvious that stuff like this just operates on a completely different plane. We can try to kinda mush it into something that we can sorta make sense of, but in reality its as incomprehensible to us as imagining a new color. Its just beyond the ability of our brains. We can understand it, use equations, but in the end were basically just interpreting something that we can never really perceive the truth of.

[@commenteroftruth9790]

You should also consider that if something seems impossible to figure out; that you dont have a solid enough grasp on what it is youre talking about. Such as photons. Theyve never been measured as a thing. Its just a name given to the phenomena of light/energy being produced. Although the reason photons as a concept exist, is because people seem to believe they must attribute the phenomena of energy conversion/light production to a particle in it of itself. Instead perhaps it is simply a product of fundamental aspects of how things work, that energy is produced. Energy in it of itself, not photons. Instead of photons being a thing that are produced, "light" is "produced" by the processes of universal laws. Light in itself isnt a thing. It isnt a particle. It is a function of equations of electromagnetic fields. It isnt a product or an addition, simply a piece of an equation which is everpresent. I.e. the equation cant exist without the entirety of the equation. You cant have X and T interact without "producing" Z. Producing is used lightly, as it isnt a product, it is simply a piece of a law which exists simultaneously along the other pieces of the law, and none of the pieces can exist without having that interaction being a whole of all of the individual aspects.

[@kedwardsTWO]

danm wished i saw this before i sat my as level exam last week

[@THEFIRSTCRAZYGAMER]

Lazers are created when recycled plastics are converted into light photons, creating ultra strong lasers actually. It can create many colors.

[@Whoaman555]

I believe this is stop motion and some good video editing software.

[@gloom_slug]

even before the second photon exists. this is why science is exiting.

[@lnadhirah]

Make one vid on semi conductors :D

[@ajohpv7033]

Subscribed

[@amas2010]

thanks!

[@saurabhs1996]

You should create ur own website about these things

[@shittinator]

4 & 8 in the dozenal system! Yup, I just brought numberphile into minutephysics!

[@WhiteKillerRabbits]

Awesome video, as always! But this is only a theory though....? What aren't we definite about in regards to lasers?

[@quesuerte1461]

I'm glad Jesse Eisenberg has decided to expand his hobbies since the Social Network

[@TheReaverOfDarkness]

I think you might be talking about some of the "microscopic" dust particles that you can't see in normal light but that become just barely visible when brightly illuminated.

[@the-thane]

1:07 Can anyone find meee, Somebody to looooove!

[@DARKEYE0808]

You should make a video, explaining what photons are

[@Tbird761]

Atoms have diameters measured in picometers, so you would never see one no matter how excited it was. It could be undergoing fission and you still wouldn't be able to notice. I'm not sure if your question was meant to be serious. The "static" you see is caused by imperfections in the laser emitter's optics. Some of the light is scattered instead of perfectly focused. Cheaper lasers will exhibit this effect more strongly than more expensive, quality units.

[@MrAkilio]

for those who can't remember highschool science the atoms are excited by the amounts of quantified energy it can absorb in the form of a photon (quantified energies being the amounts of energy belonging to a jump or fall in levels of excitation). In the same way the atom can emit an equal amount of quantified energy in the form of a photon. These energies correspond to certain light wave lengths. So if you take a bunch of the same atoms which absorb and emit the same photons in exchanging the same quantified energy you'll get a bunch of the same colour of light (same wavelengths in the visible light spectrum). And this video explains how the light comes out in a beam.

[@loon2022]

can u do explain what is optical gain in laser?

[@Calvinang93]

What you mentioned there seems to be referring to spontaneous emission. I'm confused about the how a passing photon causes emission.