What Happens to Lasers Underwater? (Total internal reflection) - Smarter Every Day 219

• Hey, it's me, Destin. Welcome back to Smarter Every Day. If you can't tell, I am on a kayak here. And we have a lot of the kayak. I guess it was a week ago I uploaded a video to the second channel. And we were trying to fish, right Trent?
• [Trent] Yeah.
• Oh, you're over there.
• [Trent] I'm over here.

So we were fishing because we've been working hard, and we just wanted to relax. The fish weren't biting. We're out here on the kayaks, and we decided to just play with our headlamps in the water. There's milfoil all in the water here. And we discovered something really, really fun with our headlamps underwater here. And I'll just let you watch a little bit of that video.

• [Destin] Okay, you ready to show them? Get your flashlight, go to the other side. So we figured this out. Put your light in the water. And you could see the beam, right? Now stick your arm further underwater. And aim up, and it bounces off the top of the water. How cool is that? Neat. Okay, now if you go too far, it'll break through. That's cool, there's a thing called Snell's window. You know about Snell's window?
• [Trent] Nah uh.

If you're scuba diving, for example, and you're in the water, and you look up, you could see the sun. But you could see it from a certain angle. So there's this window right above you where you could see the light coming through it. Beyond that, the light reflects out. And so what Trent, student over there, has to do with the index of refraction of water. Can I try it?

• [Trent] I'm moving with the- Yeah.
• [Destin] Here, trade the lights with me. Here's the camera.
• [Trent] Oh dude, that looks great from over here.
• [Destin] Isn't that neat?

And then it should just bust through. Look at that. You could see the beam goes through.

• [Trent] Dude, it bends more with the-

Did you see what happens though? Like, half the beam punches through the water. Half the beam doesn't, watch. So the beam in the water goes dark. So coming up, up, up, then it starts to go dark. And the whole thing punches through.

• [Trent] Whoa.

Okay, fast forward to now. We've got lasers now because some people said in the comments that it would be really cool if we did this with lasers. Do you have those laser goggles? I gotta say this is the only time I've ever worn lasers. Haha, laser goggles. We look like goobers, man. Alright, so here we go. We got a green laser. It's not waterproof, so I have it in a bag. For a 532 nanometer laser. And I'm wearing 532 nanometer goggles. Which means I can't see this. I have no idea what's going on. But if I do this, okay I can see that that's green. So what we're going to do is we're going to continue to play with words I could not pull from my brain in the last episode, which was: Total Internal Reflection. That's what's going on here.

So we're going to play with Total Internal Reflection. I have a bunch of goodies in my kayak. Okay, you ready?

• [Trent] You have no idea how goofy you look.

This makes so much- I love it so much! Alright, so first up. Oh, that's heavy, it's making my kayak move. Okay, I have a sheet of acrylic. It's moving my kayak 'cause it's so big. I'm cutting the light off. Gah-lee, my nasty mustache. So if we shine this laser into- Oh, that looks red. I could see in the viewfinder. If I shine this laser into the side of this square acrylic block, you can see if I'm at a 90-degree angle to the block, the laser just goes straight through and bounces off the other side and diffuses a little bit, right? But what happens if I angle this laser and I bounce it?

Look at that, the whole block lights up. And the reason it lights up is because that angle up top is an acute angle and all that light bounces around on the inside. Dude, this works so much better than I thought it would. Alright, so isn't that neat?

Okay, so if we go down- Let's go down and see if we can light this block up. Let's do it, boom, we hit the corner. Look at that, stuff starts bouncing around. So this Total Internal Reflection is a thing. So if we just go straight across, you know. Some stays in, some stays out, but-

Oh, look at that, what just happened? Oh my gosh, I'm bouncing it back and forth. Did you see that? You can't see that, can you? Do you see that? What's happening? It's bouncing back and forth on the inside.

• [Trent] I can see it with my goggles over here.

You could see it with your goggles? Okay, here we're going to do it this way. Boom, right there. You see it bouncing back and forth? Can you see it? Can you zoom in?

• [Trent] It's really hard to see it on camera.

Is it really? How do you know? You got laser goggles on, dude.

• [Trent] I can see through the viewfinder.

So, long story short, if you could bounce a laser underneath the angle of incidence, then you can get what's called Total Internal Reflection. And this happens when you have two transparent materials. Two transparent materials like air and water or acrylic and air or acrylic and water.

Okay, so that's acrylic. Let's move on to the next thing. Do we have the fiber optics?

• [Trent] Yes, we do.

Let's get the fiber optics on the kayak. Okay, Total Internal Reflection, you totally- Oh, I don't need these. You totally know what's up with that. So we get a flashlight, we got fiber optics. I can't believe we got all this stuff on the kayaks. So you know that that's how that works, right? So you got the light that's going into the fibers and it's bouncing around on the inside of the fiber, and it's going all the way through.

You know what's about to happen. You know what's about to happen. Where's my laser? (laughs) Oh shoot! Alright, here we go. Laser, look at that. Same thing, except I can't see it because I have laser goggles on. But as I move around, I'm assuming that it's going through the fibers all the way to the end. And the reason it's able to do that is it's bouncing all the way down the fibers like a pipe. Total Internal Reflection. This is all stuff you know. This isn't new stuff. So we're just demonstrating that because it's awesome.

What happens if I shine these fiber-optic lasers into the water? Oh, it kind of looks like a laser flashlight inside that. Okay, okay, alright, so now the way that fiber-optic works is really interesting. I happen to bring a bigger version of this fiber-optic with me. This is a large dowel of acrylic because obviously this is what you take on a kayak. So the way this works- I'm going to cut this off. This is so going to look like a lightsaber, dude. Oh golly. Are you ready for this? Ready, one, two. Whoa.

• [Trent] Put your goggles on.

Oh yeah, my goggles, I'm sorry dude. Thank you. Oh man. Does it look awesome?

• [Trent] It does look amazing.

Okay, can you see through?

• [Trent] I could only see it through the camera.

Yeah, because we're being safe.

• [Trent] It looks incredible.

Okay, so the reason this looks like a gigantic fiber-optic is because it is. You could tell that it's an optic 'cause of the way it is. Alright, so you've got this low angle of incidence here, it's amazing.

• [Trent] Put it in the water.

Can I light up the whole water? I can't see that. Is it lighting up the water?

• [Trent] We're about to look. Oh my god.

Is it lighting up the water? I've taken up- It's like a big flashlight at the end. I don't care, I'm going to quit talking and just do stuff. You could make a camera that would see all the way to the bottom, couldn't you? It's a light pipe.

• [Trent] A light pipe.

You could totally see if it was polished on both ends, you could make a camera that could go all the way to the bottom. Alright, we brought a gigantic piece of acrylic on a kayak. So we've done the square acrylic. We've done the fiber optics, both small and large. It's time for the Christmas ornament.

Okay, we have a Christmas ornament and we've got murky water below. We're going to need goggles for this one. Oh, that's acoustically pleasing. You want to hear that. (bubbling) That was really nice so- Oh, we got some milfoil in there. Oh, it's that's gonna- That's gonna make it more interesting. Just dudes on boats with lasers, that's all this is. Freakin' lasers on kayaks, that's all this video is. Alright, so here we go. We should be able to see the light go straight through. And I have no idea what's- I can barely see it. You could barely see it.

But what should happen- I'm going to turn this off here. But what should happen is if I change the angle inside we should get some weird bouncing that's going on, right? Yeah, we do, let's get up here close. So we're straight here. So we're firing straight inside right there and if I start to bounce things off the inside by shooting it like across the cord instead of the diameter, we get all kinds of weirdness.

What would be really cool is if we could take a shadow graph of this, if we could like blast this onto a large wall we would see all the microbes and little things living inside there. Internal Reflection does not have to be total. So if you meet this interface between the glass and the air, for example, some of the light goes out, some of the light stays in, and so that's why Total Internal Reflection is so cool.

Oh look at that, when I hit that seam- You see that? When I hit the meniscus or whatever that's called at the top where it curves, it just totally blasts the whole bulb. You see what I'm saying? It makes a huge globe, so boom. I hit the curve, lights up straight across, doesn't light up.

That's fun, we're having fun. We're learning things. We're learning things on kayaks with lasers. Okay, so now it's time for more good sounds, here we go. I think it's time. You think it's time?

• [Trent] It's time.

Laser underwater. We're going to see if we can bounce the laser up off the bottom of the water. Here's the Christmas ornament. Everybody knows what's going to happen. If the angle that hits the top of the water from underwater is less than 50 degrees, then it'll bounce back into the water. But if it's above that, it'll punch out. I just want to see it from underwater.

Okay, here we go. Tilt it back down at some point, all of the light should bounce underwater. You should see the beam get lighter and lighter as I pointed up more that reflected beam. So as it's laying down, you should see full laser beam in the water. As I tilt it up and it bounces off the top, that reflected beam should get lighter and lighter as I go up, and then at the maximum angle, once it hits the critical angle, the one inside should go dark and all the light should come straight out. Which is completely rad.

Alright, so in this video we talked about the ability of light to bounce on the inside of an object in a certain way called Total Internal Reflection. I would like to tell you about a book that made me internally reflect as well; it's called Just Mercy by Bryan Stevenson. This episode of Smarter Every Day is sponsored by Audible. And I just really want you to read this book.

Bryan Stevenson is the guy that decided to be the lawyer for Anthony Ray Hinton, who was wrongly convicted of murder in Alabama, sentenced to death row. Bryan Stevenson came to his aid and helped Anthony Ray Hinton get out of death row. It's an amazing book. I loved it. It made me reflect internally about my role in society and how I could help those without a voice.

So please go check this out by going to Audible.com/Smarter or texting the word "smarter" to 500-500. The first book is free. You can get two free audible originals. This is a big book for me. I mean, in terms of making me think about my place in the world and what kinds of things I could do to help others. So again, you could get a free book or a free 30-day trial in Audible by going to Audible.com/Smarter or texting the word "Smarter" to 500-500.

I highly recommend this book. You will totally dig it. So anyway, thank you for enjoying Total Internal Reflection time with me here on the river. Yeah, that's amazing. That's really cool stuff, so I hope you enjoyed that.

Last thing, feel free to unsubscribe to Smarter Every Day if you feel like this video earned it. By unsubscribe I mean you could unsubscribe and then resubscribe maybe, if it earned it. And then click the bell to turn notifications on. If not, that's no big deal but I hope you enjoyed this. I mean like playing with Total Internal Reflections on a kayak, it's just absolutely awesome, so I hope you enjoyed this half as much as I do.

I'm Destin, again feel free to subscribe. Getting smarter every day. Have a good one, bye.