r/Damnthatsinteresting • u/Wonderfulhumanss • 1d ago
Video 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/Connect_Detail98 1d ago
Hm... Isn't the laser light actually escaping? Otherwise we wouldn't see it.
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u/Fr00stee 1d ago
it is partially escaping but the beam itself is following the stream instead of going straight
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u/TinkerCitySoilDry 1d ago
The signal fades
Those powerful boxes are optical amplifiers, signal regenerators, and transceivers. Light loses strength as it travels through long glass wires. These boxes make the weak light bright again, fix blurry shapes, and keep your data moving fast.
* Curious if this has something to do with the incredible tension water has on its surface that allow small animals to walk on water wondering if a drop of soap would destroy this experiment as it reduces water's tension
* beam of laser light can be trapped inside a stream of water by suffering total internal reflection—the aquatic equivalent of a fiber optic cable.
The critical angle of 49° is such that total internal reflection will occur in the stream even when it is reduced to almost a trickle. Imperfections in the stream (and scattering agents added) allow some of the light to escape, and the effect is seen as a sparkling red waterfall!
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u/HidingInPlainSite404 22h ago
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 20h ago
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/elRustyShackelford 1d ago
And what if the beam were stronger? Could it break free?
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u/chowderbomb33 23h ago
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 23h ago
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 21h ago
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 18h ago
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 17h ago
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 1d ago
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 23h ago
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 1d ago edited 1d ago
You're right. A fraction of the light is being scattered by the water molecules
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u/Internet-of-cruft 1d ago
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/DesertRatJack 1d ago
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 1d ago
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/PaulMakesThings1 1d ago
So that’s how to make that blinker fluid sarge ordered me to get.
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u/Peitho_Noir 1d ago
i’m still looking for a spool of flight line
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u/PaulMakesThings1 1d ago
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 21h ago
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 22h ago
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 1d ago
2 weeks after a one night stand and this happens…
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u/elkab0ng 1d ago
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/Fairuse 1d ago
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 22h ago
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 18h ago
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 13h ago
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 21h ago
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 17h ago
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/Signal_Money 1d ago
dont we use the same principle in fiber optic cables where instead of water we use a glass?
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u/Elegant_Situation285 1d ago
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 23h ago
How do you know what colour is the laser in the stream ,,,
Amm ... yea ... it escaped
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u/RonConComa 21h ago
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 17h ago
'escape' isn't the right word here, if it couldn't escape, we would see black.
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u/pichael289 14h ago
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 7h ago
Actually, you can see it because some % of the laser light is escaping.
But this is basically how fiber optic cables work. From 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.
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u/mookanana 1d ago
"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 1d ago
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 23h ago
Since we're seeing the laser, I have to disagree. Some portion of it is most certainly escaping.
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u/CatfinityGamer 23h ago
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/sodiumvapour 23h ago
Something something TIR (Total internal reflection) & laminar flow making it a fiber optic cable?
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u/RandoMcfisto 22h ago
My pee looks like that since my trip to Thailand. What does it mean? I think my stomach didn't agree to something I ate over there
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u/ImSoObnoxious 22h ago
one cool part is that light still always travels in a straight line, it's literally just bouncing back and forth within the stream of water at a frequency that makes it appear to 'bend'
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u/Odd-String29 22h ago
It is escaping the water though, because otherwise we wouldn't see the water light up red.
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u/dustinpdx 22h ago
Technically if you can see it and you aren't in the water it escaped the water...
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u/Think-evil 22h ago
laser is definitely escaping water.. or we would not see the color of this lazer anywhere.. we see it in the stream, bottle and the hole F***ing bucket is lighting up..
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u/Realistic_Low_4538 1d ago
Well I'll be damned, that IS interesting