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  • Hi. It's Mr. Andersen and this AP Physics essentials video 13. It is on wave-particle

  • duality. And before we talk about that we should identify what a particle is and what

  • a wave is. They are both ways that we can transfer energy from one point to another.

  • And so they both would allow me to knock this can over. Let me use a cannon ball first.

  • And so that is a particle. I am transferring energy through a particle from point A to

  • point B. The cannon ball is going from one point to another and I have knocked the can

  • off. Now how else can I knock it off? Using a wave, well you see that there is a chain

  • here. I could add energy to it at one side. That would create a wave that would knock

  • the can off. Now you can see the chain is not moving. That the wave continues to move

  • back and forth. It is transferring energy through the medium. And so objects can either

  • be particles, like a baseball or they can be waves, like sounds waves. You are listening

  • to sound waves right now coming from the speaker on your computer or the speaker in your headphones.

  • How did it get there? Well I vibrated the air. My microphone picked up on that. And

  • then your speakers vibrate in the air. But it is not like sound is moving from me to

  • you. It is just vibrating the air. And so what scientists have found is that there is

  • this duality. Particles act like waves and waves act like particles. And in this video

  • we are going to talk about how particles have wave properties. And this was first seen in

  • the double slit experiment, which is kind of counter-intuitive. But it will show you

  • what is going on in the world of the very small. The world of quantum mechanics. And

  • so let's set this double slit experiment up. What we have are two slits on this side. And

  • what we are going to do is shoot particles at it. So you could imagine that we are shooting

  • baseballs or bullets or marbles or even spray paint that are going through those two slits.

  • And so what do you think will happen? Well those particles are going to move through

  • those slits and they are going to hit some kind of a screen. Now what do you think it

  • is going to look like on that screen if I turn it so you can see it? Well you probably

  • guessed this. In other words the particles are not all moving in a straight line. But

  • they are moving through the slit. They are hitting this screen and then we are seeing

  • that if we turn it sideways. And so what do you think happens if we were to close up one

  • of these? Well the particles are not going to make their way through. If we close this

  • one what is going to happen? Particles are not going to make it through there as well.

  • And so if we remove both then we are back to where we started. So you should not have

  • learned anything new. This is how the world works. Now let's say we are using waves now.

  • And so imagine instead of shooting spray paint through these slits we now have got a wave

  • tank where we are generating waves on the left side. Those waves are moving through

  • the two slits. And then we have detectors on the screen that are showing us what is

  • the amplitude of those waves. How much energy is being transferred through? So if we kind

  • of simulate that it is going to look like this. And so what we are seeing is interference.

  • You can see that as those waves move through we are increasing the waves in some places,

  • decreasing the waves in some places. And what you see is an interference pattern that would

  • look like this. Because those are moving as waves they can interfere with each other.

  • And so you do not have two discrete units. That is how waves look in a double split experiment.

  • And so then scientists said well let's look at something very, very small. Let's look

  • at an electron, which we tend to think of as being a particle. And so let's shoot particles

  • through this double slit experiment and see what we see on the other side. So you might

  • image they are going to move through. If we turn it towards you might think it would look

  • just like those marbles or that spray paint did. But what you see is an interference pattern.

  • So we see these things that we think of as particles and they are starting to act as

  • waves. They are interfering with one another. And so scientists thought well maybe they

  • are all going every which way. And so they are interfering with each. What if we just

  • shoot them one at a time. So let's just shoot a few electrons at a time. And what they find

  • is as they do that over minutes and minutes and then hours and then hours, what is happening

  • is this interference pattern starts to emerge. And so this is crazy to scientists. So what

  • is going on. Are they somehow interfering with other electrons in the future or in the

  • past? Are they somehow dividing in half and then interfering with each other, those two

  • parts of that electron? And so scientists want to test that. So what they do is they

  • observe it. In other words they put flashes of light that are going to be released when

  • they incident with an electron and they see what happens these to see which one of these

  • holes is the electron going through. And as they start to observe it something even weirder

  • happens. It goes back to acting like a particle again. And so again, what is this showing

  • us? That these particles that we think of as discrete little units are actually behaving

  • like waves. And it opens up this world of quantum mechanics. And so you could do this

  • if you wanted to. This works with any kind of a light source, like a laser light source

  • where it is moving in one coherent plane. And so what you could do is just shine a laser

  • through these two slits. You could do this is a physics classroom. And what you will

  • get is this diffraction pattern on the other side. It is something that we can observe

  • but it is being created at the quantum level. And what do I mean by that? Well we live in

  • the world classical mechanics. We live in a world where everything travels much less

  • than the speed of light. And things are much larger than the size of an atom. And so we

  • live in this world of classical mechanics. That is mostly what you are going to learn

  • in Physics I and II. But know this, that as things get really really fast we have to adjust

  • and use relativistic mechanics. And as things get really, really small we have to use quantum

  • mechanics. And so the double slit experiment does not work in our classical mechanics world.

  • But it does work in the world of the very very small. And so did you learn this, how

  • classical particles like electrons and photons can have wave-like properties? I hope so and

  • I hope that was helpful.

Hi. It's Mr. Andersen and this AP Physics essentials video 13. It is on wave-particle


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波動粒子の二元性 - その1 (Wave-Particle Duality - Part 1)

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    羅紹桀 に公開 2021 年 01 月 14 日