r/Damnthatsinteresting • u/Wonderfulhumanss • Jul 27 '26
Video This laser can't escape the water. Physics won't let it.
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u/Wonderfulhumanss Jul 27 '26
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/Connect_Detail98 Jul 27 '26
Hm... Isn't the laser light actually escaping? Otherwise we wouldn't see it.
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u/Fr00stee Jul 27 '26
it is partially escaping but the beam itself is following the stream instead of going straight
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Jul 27 '26
[removed] — view removed comment
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u/HidingInPlainSite404 Jul 27 '26
Light attenuation in those wires doesn't happen quickly. They can go like over 50 miles without needing amplification.
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u/ormr_kin Jul 27 '26
depends on wavelength too. 1310nm can only go like 10km depending on the spec of the optic. 1550nn can often go up to 80km.
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u/adoodle83 Jul 28 '26
iirc, in theory, the wavelength doesn’t matter as much for distance. in practice, due to physical cable manufacturing limitations, the wavelengths do have an impact, but not substantially. the distance is controlled by the type of laser and intensity. a lot of this is semi-arbitrarially defined by what came into industry popularity and adoption.
The type of Fibre (SingleMode Fiber vs MultiMode Fiber) also makes a substantial impact. SMF is almost exclusively used for any long distance transmissions.
you can get FiberStore to make optical transceivers in any wavelength and optical cables to almost any specs for reasonable costs. i had a custom 75m 12-strand MTO/MTP cable for $300
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u/elRustyShackelford Jul 27 '26
And what if the beam were stronger? Could it break free?
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u/chowderbomb33 Jul 27 '26
no, beam "strength" is not a factor. The refractive index differential and incident angle is what matters. Water is optically denser than air and thus when it goes from a higher refractive index to lower exceeding a critical angle, it can undergo "total internal reflection".
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u/robodrew Jul 27 '26
Eventually though with enough strength I would think the laser would contain enough energy to cause the water to boil, allowing it to escape
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u/Lunarvolo Jul 27 '26
It depends, you'd need to define stronger.
Decreasing the wavelength with same amplitude (Increase in power/making it stronger) would change colors and change it's angle of refraction and it would break free/change.
Larger amplitude, same wavelength you're increasing the energy which can cause change in temperature which affects refraction lightly, boil would absolutely change it, and do on
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u/wonderwall879 Jul 27 '26
I am thoroughly impressed with these correct answers in the thread. Great treat while im literally working on fiber right now lol.
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u/kylo-ren Jul 27 '26
The beam is not a single ray of light. It's not being curved. It's a bundle of light rays that are repeatedly reflected by the stream boundary. Some light scatters out when it is not reflected back inside and reaches the camera sensors and our eyes.
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u/MegaDonkeyDonkey Jul 27 '26
This is how fiber optic works as well. If you see the color in the housing then it's leaking/emitting so yeah, you are 💯 correct.
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u/mew_rei Jul 27 '26
Yup, we use a tool called a VFL (Visual Fault Locator), which is basically a visible spectrum laser with fancy connectors on the end, to troubleshoot fiber and if it lights up like a Christmas tree, you found a fault. Much cheaper than an OTDR, albeit you have to be able to visually inspect the suspect area.
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u/SeekingAnonymity107 Jul 27 '26 edited Jul 27 '26
You're right. A fraction of the light is being scattered by the water molecules
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u/KrimxonRath Jul 27 '26
Their right what?
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u/SeekingAnonymity107 Jul 27 '26
I object to erroneous autocorrects too. Corrected.
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u/DesertRatJack Jul 27 '26
What about when the stream starts sputtering? Will droplets contain the laser but it will escape when its air in between?
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u/Pretty_Eater Jul 27 '26
I'm guessing a combination of the first sputter creating an air barrier, and also there is still some energy leaking out anyway, that's why we can see the laser at all.
So it's not like you can turn a laser on and store it in water indefinitely.
But I am also super baked and super unqualified to answer this so someone will probably correct me.
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u/Internet-of-cruft Jul 27 '26
This is the same thing as fiber optic cable.
Just seems more interesting because we don't normally associate water as being a waveguide the same way glass is.
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u/discofunkbunny Jul 27 '26
Could this be used as a principle for a lazer defense system?
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u/PaulMakesThings1 Jul 27 '26
So that’s how to make that blinker fluid sarge ordered me to get.
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u/Peitho_Noir Jul 27 '26
i’m still looking for a spool of flight line
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u/PaulMakesThings1 Jul 27 '26
Yeah, and even if I can find a new cartridge of honks for his jeep I have no idea where to load it.
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u/kidmeatball Jul 27 '26
I'll help you find it if you can help me find some ice mix. All the bars in the area are out and our ice machine needs to be refilled!
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u/dandroid126 Jul 27 '26
Sarge is gonna be pissed when you show up with blinker fluid when he clearly told you to get headlight fluid.
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u/Smart-Firefighter87 Jul 27 '26
2 weeks after a one night stand and this happens…
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u/elkab0ng Jul 27 '26
This is basically multimode fiber! Same principle, just a little more leaky
Single-mode, used for longer distances, would use a much thinner stream (fiber) and a slightly different jacketing, so that much less light would bounce around internally and leak out the sides.
We also use cable identifier illuminators which are basically very “leaky” lasers to deliberately do this - make lots of visible light “escape” from the fiber to visually identify it in a data center, usually because SOME CABLE RAT mislabeled it 😂
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u/Uberpastamancer Jul 27 '26
The fact that we can see it is proof that some of it is escaping
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u/ayypecs Jul 28 '26
correct, it's slightly pedantic but OP really meant that the overall beam's trajectory is now following the flow of water
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u/Fairuse Jul 27 '26
The fact that you see the stream of water glow the color of the laser means the laser is escaping.
Fiber optics are designed to achieve near 100% internal reflection (i.e. trap light inside). Thus when you shine laser into one end of a fiberoptic cable, you don't see the laser through out the length of the fiberoptic. You'll only see the laser exit at the end tip of the fiberoptics.
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u/Avalonians Jul 27 '26
To be accurate, the reason you see the laser isn't because the water/air surface isn't reflective enough, but because of light scattering inside the water.
Optic fibers will obviously be manufactured to prevent as much scattering as possible (though it will be impossible to achieve perfect transmission in normal temperature conditions) but the way the light reflects on the edge of the stream is exactly the same here and in a multi-mode fiber. There isn't a single shred of light that can go out of the water if it hits the edge at an angle above the critical angle.
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u/eternityXclock Jul 27 '26
it actually does escape the water. if the laser would be confined to the water we wouldnt be able to see the red colour at all. this red colouring appears because parts of the laser beam are still refracted to our eyes.
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u/BillCuttingsOn Jul 27 '26
Did scientists ever go down the road of considering water filled lines for information transfer instead of fibre optic? I’m sure it’s impractical, but curious nonetheless.
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u/E-2theRescue Jul 27 '26
If it won't let it escape, then how come we can see it in the water?
*Taps forehead*
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u/North_Affect_8167 Jul 27 '26
The laser is escaping the water you fools! The light is reaching the camera sensor, thus light is emitting towards the camera and not stuck in the water stream.
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u/Elegant_Situation285 Jul 27 '26
the laser should have the right to choose for itself. this is truly horrific.
edit: for those who keep dm'ing me, yes i believe photons are just trapped angels. that's just science.
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u/Giorgist Jul 27 '26
How do you know what colour is the laser in the stream ,,,
Amm ... yea ... it escaped
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u/RonConComa Jul 27 '26
well, some light is escaping, else you won't see the the stream glow red. but the majority is trapped like in a fiber optic cable
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u/expected_69 Jul 27 '26
'escape' isn't the right word here, if it couldn't escape, we would see black.
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u/pichael289 Jul 27 '26
The fact your seeing it means light is escaping. Refraction bounces it around the water, it's why the speed of light is slightly lower in water, it's not actually slowing down it just takes longer to get through.
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u/Diz7 Jul 27 '26 edited Jul 29 '26
Actually, you can see it because some % of the laser light is escaping.
But this is basically how fiber optic cables work. From the angle of looking the inside of the water/fiber the outside layer looks like a mirror, it just bounces down like it was a mirrored tunnel, until it hits a curve that bends too sharply and the mirror effect fails. It's like looking at a lake and it looks like a mirror, but when you get close and look straight down you can see through it, light below a certain angle bounces off but head on it goes right through.
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u/ZippoS Interested Jul 27 '26
Well, someone of the light is escaping the water. Because we can see it.
But yes, most of it is bouncing back and forth along the stream like it would in a fibre optic cable.
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u/mookanana Jul 27 '26
"Just so you know, it's NOT that common, it DOESN'T happen to every guy, and it IS a big deal!"
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u/Iokua113 Jul 27 '26
That's kinda what my pee looked like after I got a 1.1 CM long kidney stone blasted earlier this month.
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u/generally_unsuitable Jul 27 '26
Since we're seeing the laser, I have to disagree. Some portion of it is most certainly escaping.
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u/CatfinityGamer Jul 27 '26
The light literally is escaping if we can see it. It's just that most of it doesn't escape.
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u/Realistic_Low_4538 Jul 27 '26
Well I'll be damned, that IS interesting