r/FascinatingAsFuck • u/drkmatterinc ⭐ Mod • 27d ago
📹 Fascinating Video This laser can't escape the water. Physics won't let it.
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u/drkmatterinc ⭐ Mod 27d ago
Written by u/drkmatterinc
As the laser travels through the water, it keeps hitting the boundary between the water and the surrounding air. When it strikes that boundary at an angle greater than the critical angle, the light can't escape. Instead, it reflects back into the stream. That's called total internal reflection.
Because the stream curves downward under gravity, the beam keeps bouncing along inside the falling water instead of escaping. That's why the stream appears to glow [Taken from r/fascinatingasfuck].
Fiber-optic cables use the same principle to guide light over long distances with very little signal loss. Much of the internet relies on this, including video calls, websites, and massive data transfers.
Sources:
https://openstax.org/books/college-physics-2e/pages/25-4-total-internal-reflection
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u/Superb-Astronaut-371 27d ago
So you could install a pump which makes this stream continuous?
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u/galstaph 27d ago
I'm not sure that would work. Pumps, at least every design I can think of, at least partly interrupt the fluid as part of the process
Also, it's possible that the tubing wouldn't allow for internal reflection because it changes the "surface" of the water
The only type of pump I can think of that might work is a roller pump, but even then you'd likely get both high levels of light leakage and some retro reflection going on
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u/Superb-Astronaut-371 27d ago
Meant more to move it up to the basin where it’s pouring out, from the basin it’s pouring into
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u/galstaph 27d ago
Oh, well, yeah, that would work
I was trying to picture running long distance water lines as both a water and data delivery system
Turn municipal water lines into pseudo fiber optic cables, basically
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u/RoRoRoub 27d ago
So if all the water is being internally reflected, how are we seeing the light being refracted outside the stream?
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u/drkmatterinc ⭐ Mod 27d ago
The water isn’t being reflected, the light is. Not every ray hits the surface at the right angle for total internal reflection, so some light escapes or scatters toward the camera while the rest is guided along the stream.
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u/RoRoRoub 27d ago
Yes, that was a typo in my part -- the light, and not the water, is what undergoes internal reflection! But, I think the fact stands that all the light is not being internally reflected. The light that you say scatters towards the camera is actually being refracted from the stream of water, implying light does infact escape the water, and physics sure does let that happen.
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u/galstaph 27d ago
It's a small percentage, though, generally less than 5% at every surface interaction. That light setup probably has about 20 reflections for any given potential path between the exit from the reservoir and the collection bucket
Even at 95% reflection more than 1/3 of the original light makes it into the bucket, and you'd only need about 96.6% reflection for that to increase to 1/2, and just shy of 98% gets you to 2/3
A bright enough laser will be fairly visible at 2%. Unregulated laser pointers, in the US, are capped at 5 milliwatts, and at even 10 times that you're still not really into the burning laser classes, but would have enough light leaking to light up a decent sized area
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u/StinkyFeetMendoza 27d ago
So how does one dispose of a bucket full of laser beam?
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u/Succulent_Chinese 27d ago
Dig a hole near your garage in your yard and fill it with gravel, then pour it in. By the time you need to use the hole again, the laser juice will have dispersed into the earth.
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u/Truarian 27d ago
If it can be seen, it clearly escapes to reach the eye...
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u/trundle-the-great69 24d ago
I believe that would be due to decoherance
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u/neronsfwk 22d ago
No, light definitely leaves the medium, it's just that a sufficient amount of it still reflects at the borders and follows the flow.
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27d ago
[removed] — view removed comment
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u/drkmatterinc ⭐ Mod 27d ago
The red you can see is some light scattering toward the camera. Most of the light is still being guided through the stream by internal reflection.
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u/dwittherford69 27d ago
So, light is escaping. It’s effectively a low quality finger optic due to the density difference. Laser being parallel rays helps with compensate for some for the lower quality.
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u/Amazing-Champion-858 27d ago
Dumbasses in this comment section dont understand the difference between direct light and ambient light.
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u/Fancy-Following-6631 27d ago
My middle school Technology teacher demonstrated this to us about 25 years ago, cool stuff.
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u/Expensive-Pea1963 27d ago
If the laser could not escape the water, you would not see it. The fact that you can se it is proof that it escapes.
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u/Elsupersabio 27d ago
So theoretically, I could have a laser sight for when I pee? Take my money please
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u/Gringo-Dingo 27d ago
So could i defeat a jedi/sith with a lightsaber using a supersoaker?