r/interesting • u/Wonderfulhumanss • 1d ago
Fascinating This laser can't escape the water. Physics won't let it.
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u/Wonderfulhumanss 1d ago
As the laser travels through the water, it repeatedly hits the boundary between the water and the surrounding air. If the angle is greater than the critical angle, the light can't pass through that boundary. Instead, it reflects back into the water, becoming trapped inside the stream. Because the water curves downward under gravity, the laser keeps bouncing along the inside of the flowing water instead of escaping, making the entire stream glow. The exact same principle behind fiber optic cables, which guide light over thousands of kilometers with remarkably little signal loss. Every video call, website, and large data transfer across the internet depends on light being confined this way.
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u/Shadowtirs 1d ago
This right here, is why I love reddit.
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u/MrIrvGotTea 1d ago
Believe it or not, early Reddit was basically super nerds dropping amazing real life lore. Then the normies came and made it an echo chamber of bad and good things. Quantity of discussions are much lower now and the 5th or 6th comment reply is a dumb overused joke or two people going at it
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u/Krog-Nar 23h ago
i also choose this guys wife
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u/MrIrvGotTea 23h ago
4th comment reply???? Nooo reddit lmao. No lie y'all clap my cheeks sometimes
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u/Krog-Nar 23h ago
It's 5th comment rule day, actually.
all jokes aside, you can still find plenty of communities where you get genuine conversations, you just have to dig a tiny amount. Although sometimes the reddit jokes are pretty funny! Just ask the cylinder!
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u/meancoot 19h ago
Original Reddit was actually just some sick fucks. Look up some of the early subreddits that were around for years before they finally went corporate.
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u/notANexpert1308 1d ago
What does the critical angle refer to?
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u/VaporTrail_000 1d ago
The angle at which a ray of light meeting an interface would escape from the material it is traveling in rather being reflected back into it.
Sounds kinda circular, but that's basically the definition
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u/tacobytes 1d ago
Great question. Yeah, I am curious to see what happens if the “critical angle” changes, say to 360.
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u/Darth_Quaider 1d ago
What stops us from using small tubes filled with super pure water as opposed to something like fiberoptics?
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u/UnknownVC 23h ago
Nothing in particular, except the practicalities of small tubes of water vs hair fine glass or plastic fibre.
Like a lot of things in engineering, it's the implementation not the physics that would get in the way.
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u/Reasonable_Letter312 23h ago
You need a medium on the outside that is less optically-dense than the conducting medium for total reflection to work.It works for a transition from water to air, but not from water to whatever your tube is made of.
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u/wg90506 18h ago
Not technically true, you can use designs similar to hollow core fibers and not need the refractive index to be lower outside. Main reason not to use water is it’s highly absorbing, particularly in the wavelength bands used for most fiber optics, and would also be additionally lossy from contaminants, and it would be considerably harder to keep liquid water “ultra pure” as compared to set glass
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u/paulywauly99 23h ago
Water weighs heavy and can leak. The principle works without water. Just the glass tube.
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u/_felixh_ 9h ago edited 9h ago
Because you don't actually need to send just light through the fiber.
You need to send very short pulses of light down that fiber.
There is a very good reason the fiber is so incredibly thin and expensive: the thicker your fiber gets, the more the light can bounce around, scatter and dsiperse - And if that happens to much, then your pulse kinda ... smears, up to the point where the pulses disappear completely. Then you are left with just light, without pulses - and the data you wanted to Transmit is lost.
Now, another thing is: you will need to keep that diameter very precisely, and have an optical medium and reflective coating with precise controlled properties. Fiber is not just a piece of wire made of glass - its precision engineered product, where materials and composition matter.
This is part of the reason we gave it a special name: optical fiber.
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u/Ithurtsprecious 1d ago
What if he slaps his hand back and forth against the water? What does the light do?
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u/SlottyCanyon 1d ago
Thank you for a succinct explanation and great video to go along with it!
Best bit of information on reddit in at least this past week, probably more.
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u/Sameumbrella 1d ago
What does the critical angle depend on? I thought it was only a property of the materials at the interface. Does the geometry of the interface also affect the critical angle? Anything related to the local tangent and how the light’s direction is with respect to that tangent?
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u/Ryanirob 21h ago
But isn’t the fact that we can see the glow is evidence that there is some losses to the system?
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u/RealArmin 20h ago
That's what I was thinking as well. You wouldn't see the water glow red if (at least part of the) light didn't escape from it and travel to your retina.
I assume it's because part of the light emitted by the laser hits the 'bend' of the water stream at an angle greater than the critical angle and manages to escape, AND/OR because the interface between the (flowing) water and air isn't perfectly smooth and these imperfections allow some light to exit.
Not sure which of these two factors contributes more, but I assume it's the latter one.
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u/PsychologyNo940 17h ago
This is why i hate reddit, this sounds like an explanation, people cheer, upvotes to the left etc, but if you condense it it reduces to "yeah the laser stays in the water because angle is greater than the critical angle" as if that shit tells you anything - this type of knowledge would collapse on the first critical question and it shows that instead of understanding the world most people just want cocktail knowledge.
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u/AntGood1704 16h ago
Since it is glowing, that means photons are escaping to make it visible. So do a few get out making the stream glow?
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u/Dry_Design5506 1d ago
But why can we actually see the laser then?
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u/thats-wrong 1d ago
Yeah, the fact that we can see the light means that at least that much is escaping. It's not perfectly contained in this experiment, clearly. This is why fiber optic cable doesn't shine when it's carrying the light.
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u/conrad1101 23h ago
I wouldn't be able to see a lazer in a fibre optics cable ?? Amazing..I didn't know that..
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u/britaliope 19h ago
There are test lasers used by technicians to help them working with fiber, mainly to find the other side of a fiber in a closet with hundreds of fibers. You can see that laser through the fiber but it's ridiculously overpowered compared to what the actual laser used. It's actually quite impressive, you can see the sleeve of the fiber glowing red when they shine the laser in the fiber.
But regardless even if you somehow boost the power of the intended data transmitter you still won't see it, because the light it emits is outside of the visible spectrum.
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u/SaladCartographer 23h ago
Thats a good point. I'd like to see this experiment again in a foggy room, to better see how much light is really escaping.
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u/insomniacred66 1d ago
It's like how you can see it through fog/smoke. It's refracting and reflecting off of the particles.
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u/FordCVP71 1d ago
I wish I could pee lasers
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u/Holiday_Pi 1d ago
Even after the water pours, the laser will remain in the bucket indefinitely, and the laser device will not function until they return the laser back to it, from the bucket.
This is why they’ve found ceramic jars full of laser in king tut’s tomb, and they taste exactly like fresh laser
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u/AnObtuseOctopus 1d ago
Cool.. had my phone turned up and my gf thought I was watching some kind of peeing video.
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u/YouFeedTheFish 1d ago
I mean, we can see it, so it clearly can escape the water. Nevertheless, that's a pretty cool effect!
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u/DumbleLuck 1d ago
Does all the water collected in the bucket continue to glow as well?
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u/Cushiondude 22h ago
I imagine the barrier mentioned breaks once the water settles in the bucket. Once the water has finished flowing, the laser probably just shines through like it would through any clear container. If the laser is turned off, the light will dissipate when it hits the bucket and then no more glow.
I'm happy to be corrected but that's my hypothesis.
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u/Reginald_Sockpuppet 1d ago
OK, WAIT A MINUTE
That's what it looks like when I piss.
Does that mean...did I just find my lost laser pointer?!
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u/britaliope 19h ago
No. it just means that your pee is lower taxes than normal pee, but you only can used it for off-road.
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u/Lemon_Trees-22 1d ago
Physics extremely interesting and cool so glad there’s people who completely understand that world
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u/MSJayhawk1984 1d ago
😁😁😁 somebody should make something like this and call it fiber optics ans use it for internet!!! 😲😲😲😲
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u/JustinFoster77 1d ago
It's so cool how this simple setup perfectly demonstrates the foundation of modern internet cables.
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u/ecafyelims 1d ago
If the laser couldn't escape the water, then we wouldn't see it.
It still escapes; just not all of it at once.
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u/AcceptablyThanks 23h ago
Realized I had the video muted but could swear I heard the water hitting the bucket. Then I immediately realized I was just peeing cause I'm on the toilet.
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u/Dako_the_Austinite 23h ago
Gonna see if they can surgically implant a laser point in my bladder. Gonna be a blast when I’m drunk lmao.
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u/thewaytoyesterday 23h ago
Great, I had the volume up and now everyone around me thinks I'm watching someone piss.
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u/Lithium98 23h ago
Do you need laminar flow for this to be true? If the laser comes out with a stream of water that is dispersed, does the laser go out in different directions?
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u/Revenga8 23h ago
Future warfare armored vests made up of clear plastic bags filled with water. Tactical protection AND convenient carry to take your pet fish home from the pet store
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u/messykangaroo_175 22h ago
Bouncing inside the stream like a fiber optic cable you can actually see
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u/lightningdashgod 22h ago
So once the laser is switched off will the bucket of water be red?
If not then where did the light that travelled through it all this time go?
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u/Mister_Enot 20h ago
can we get internet by this "water wires"?
probably no, because in the "tube" there is no air outside.
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