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The Concise RDWorks Learning Lab with Russ Sadler. Session 3: Understanding the Laser Beam.
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Well, we’ve got the science behind us now, so here we are at the machine today.
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We’re going to have a bit of fun and games with the laser beam because, hey, this is a laser machine
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after all.
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What we’re going to do is to try and relate some of the physics that we talked about last time in that boring theory session and see it in practice.
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Now, it is important that you understand the concepts that we talked about last time, because they are essential to the way in which this machine works.
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And as we come across them, I shall keep endlessly, boringly repeating them, making sure that they get drilled into there. Now,
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let’s see how much science you can relate to what I’m going to demonstrate today.
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Don’t worry, I’m going to tell you the answers. I’m not going to say too much about the laser tube itself,
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other than the fact that the laser beam is bouncing off of the mirror over in that corner there and it bounces off of a mirror here.
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And then it comes across here and bounces off a mirror here goes passes down through this lens tube here.
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And there’s a lens stuck in there to amplify the laser beam.
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And so what comes at the bottom here is a very, very concentrated, intense beam of light.
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So we’re not going to worry about the lens, all we’re going to worry about today is the laser beam itself,
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its characteristics and its strange properties. It’s just convenient that I happen to have my cup of coffee here.
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I’m going to place that underneath the nozzle there because hey, it needs heating up.
Transcript for Understanding Laser Beams (Cont…)
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So let’s have a go at heating it up. OK, I’m going to switch the laser beam on now and I’ll let you see what’s happening inside my cup of coffee.
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I’m heating my coffee up, with a laser beam.
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Woh ho, I mentioned to you that the laser beam is operating at ten point six microns wavelength, which is above our ability to see it.
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So we will not be able to see the beam when it bounces off this mirror here and hits this mirror here and goes down here and heats my coffee up.
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Let me do the same thing again. Heat my coffee up and you can hear it and see it.
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And then I’m going to do this. That was hot.
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It’s burnt its way right through that piece of card. Even though you can’t see it,
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there is a beam of energy that’s going across there. Don’t worry, I’m not going to put my finger in it because in six years it’s bitten me about twice.
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We’ve seen that A. that laser beam can heat my coffee up.
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B. it can burn its way through card. So here we’ve got a thermocouple connected up to a thermometer, a digital thermometer. So if a put that in the flame,
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you can see that the ball is getting red hot and it’s nearly 900 degrees C.
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OK, so this little thermocouple measures up to about thirteen hundred degrees C, 700,
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nearly 800 degrees C. If you remember, I said to you, there is no heat in a laser beam, there is only energy. But look, we’ve just measured it.
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It says 800 degrees C, so everything about this beam says it’s hot.
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This is why people think that laser beams are dangerous. They’re hot!
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If you remember the mechanism by which a laser beam heats up the product,
Transcript for Understanding Laser Beams (Cont…)
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everything has got molecules that are doing this and the laser beam is actually accelerating the vibration of the molecules and making them hotter.
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One of the things you have to understand about the laser beam is it’s a very,
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very small world that it, and I’m going to use this in inverted commas, “sees” it only “sees” three things.
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Now, I’m afraid you’re going to get used to my pens and bits of paper.
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I always remember, although it’s a very, very, very long time ago.
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My art teacher saying to me, for God’s sake, boy, take up plumbing, but hey, I’m only trying to write today.
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So what we’re going to do is No. 1, three things remember.
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Number 2 and number 3, let’s talk about metal to start with. Metal equals mirror.
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Every single solid metal reflects ten point six micron wavelength
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light. We have metal mirrors in this machine to make the beam of light pass around the machine and finish up down here.
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Now, we’ve got a second group of material which I call salt-like. Oh dear, hyphenated, salt-like material.
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Now, these are a special group of materials.
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A very, very small group of materials, that like this are used for lenses, because they allow the light to pass right the way through, just like glass.
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But strangely enough, glass does not let the light through.
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I know that’s puzzling. Yeah, we’ll talk about that shortly when we talk about safety.
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And then we’ve got another type of material here, which is called gallium arsenide, which looks like a mirror, but it’s not a mirror.
Transcript for Understanding Laser Beams (Cont…)
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It’s one of these special salt-like materials that allows infrared light to pass through it.
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Even though you can’t see through it, the infrared light can pass through it.
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And we use that for a lens as well. So there are two, of a very small group of materials and there’s only probably about
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another three or four materials in this group that will allow the light to pass through.
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So we talked about things that reflect, things that look like windows,
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and now group number three. I know it looks as though it’s been written by an incompetent person,
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but actually, I’m sorry, I was writing upside down and back to front almost. That’s group number three.
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Everything else, everything else becomes affected by the wavelength of this light.
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Remember what I said? The wavelength of light is busy shaking molecules.
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It doesn’t shake these, it passes right through. It doesn’t shake these, it bounces off.
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But with everything else, the light wave stimulates molecular motion and molecular motion is what? Heat.
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It generates heat. So if I stick my hand in front of that laser beam, is my hand metal, is my hand salt?
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No, everything else has molecules that can be stimulated by the light that hits them.
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And so they will heat up my hand. This pen, my coffee, the cup.
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Anything! Your eyes? Oh, we won’t talk about those yet, we’ll wait until the safety session.
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I’ve got here a piece of mild steel, which, as you can see, it’s all scratched a little bit oxidized, maybe a little bit rusty on that side.
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Now, look, I’ve got a piece of MDF there, and I’m just going to fire the laser beam at that MDF.
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Now, the beam is not focused, remember, I’ve taken the lens out.
Transcript for Understanding Laser Beams (Cont…)
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OK. Are you happy that the team is there? Like a game of pool or billiards, the beam is going to hit here and it’s going to bounce off.
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I’m going to put you inside the machine so that you can see the reflected light off that piece of pretty cruddy steel.
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Even though this is a pretty terrible surface piece of mild steel, which is one of the lower reflecting materials,
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which is why I’ve got that as a base in my in my machine here, it still reflects the light.
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I’ve got a pretty gross looking piece of ordinary aluminum, not shiny. Ready, here we go.
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OK, now you saw the difference there. How much energy was reflected because it actually got white hot in the middle there.
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You see, there’s nothing special about this copper. It’s pretty, it’s pretty grubby on the surface.
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Just give it a little pulse. Make sure. Oh, dear.
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That’s a very small pulse. Look at that. Goodness me.
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So I hope you can see how dangerous it is to have pieces of metal underneath your laser beam,
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because if it reflects at you, it’s very, very dangerous.
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So be very careful if you put metal in this machine. Now, there is a good reason why this machine has a safety cover.
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You won’t find me using it very often, because I have to do video 99 percent of the time.
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I’m working with the machine open. Surely it must be dangerous.
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We’ll talk about that in a future session about the hazards associated with the laser beam itself.
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What I want to do at the moment is just display some of the properties of the laser beam.
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We’ve seen that it definitely reflects off of metals, some more than others.
Transcript for Understanding Laser Beams (Cont…)
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We don’t need to talk about these because hey, they’re lenses, they pass the light through. The laser beam attacks only the molecules that it contacts.
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So it’s only the surface molecules that get heated up. So when I was heating my coffee up, I wasn’t actually heating my coffee up at all.
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What was actually happening was I was heating the surface molecules of the liquid, the water, basically.
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And those water molecules were immediately changing to steam,
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revealing more molecules of water underneath, which then immediately changed to steam. Heat
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never got a chance to travel into the water. Now, I like to,
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I like to imagine heat like a stack of dominoes, which when you push the first domino,
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all the others fall over because the energy from the first one knocks into the second one, knocks into the third one, et cetera, et cetera.
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If the first domino gets evaporated. It’s never going to topple over the rest of the dominoes.
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What it does is expose the second domino to be evaporated and then the third domino to be evaporated.
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And that’s exactly how a laser beam works. The light cannot penetrate any deeper than the surface that it sees.
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Now that’s a very important concept to remember. You can’t send the light into the material to damage the material in internally.
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You have to work your way in gradually from the surface, layer by layer by layer.
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That’s a difficult concept to imagine when you see the speed at which the actual damage takes place. I might be able to demonstrate that to you.
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I’m going to put the pulse on, and then I’m going to pull this away.
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So here’s where I started the burn, obviously. And then I pulled it away and as I pulled it away, so I accelerated and got faster and faster and faster.
Transcript for Understanding Laser Beams (Cont…)
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And as I got faster and faster and faster, the amount of damage got less and less and less.
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So here we were only damaging just the merest surface molecules, whereas here we were damaging the surface molecules plus a few.
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We were still burning into the material, but it was eating into it, molecule by molecule by molecule.
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Did you observe how bright and white that light was? Carbon requires 5000 degrees C to disappear into gas?
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So what we’re seeing in that white light, there is a very, very high temperature, very high temperature.
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There’s no way that there’s 5000 degrees worth of heat in that beam.
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The beam is actually heatless, but what there is sufficient energy to stimulate carbon, which I have to say stimulates very easily and quickly.
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Remember the cake and the jelly from the last session? Well, this is the jelly.
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It’s being very easily vibrated by this fairly low level of power because we’ve not even focused.
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I’ve now, put the laser beam through an amplifier. Through the lens, and I want you to watch what happens.
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Did you see that bright white light? How long did the beam last? A tenth of a second?
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Let’s have a look on the other side. Look, the laser beam has pierced
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it’s way, right through six millimeters of MDF in less than a tenth of a second.
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I’m sure there’s a SMARTY-PANTS out there somewhere that’s going to say,
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“Hang about. That’s a piece of metal. How come that didn’t reflect the light?” What’s actually happening there?
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This, although it’s pure metal, it’s not metal on the surface, there is an oxide of metal on that surface and an oxide of metal is not pure metal.
Transcript for Understanding Laser Beams (Cont…)
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So consequently, anything that has oxidized, it’s a non metal, therefore it will absorb energy.
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And that’s why the energy was absorbed into that little ball there and we were able to detect the heat.
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Paper? Well, that’s obvious. That’s non-metal. So that absorbed all the energy and burst into flames.
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So here we’ve got a fascinating little demonstration to finish up with. We’ve got a piece of acrylic here.
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I’m going to let the beam tell you what it’s doing.
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That’s 50 millimeters deep in about three seconds.
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We’ve got a lot to talk about. That’s something for the future, an explanation of what’s going on there.
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But it does just show you some of the strange properties of this laser beam.
