Wave-Particle Duality - Part 2 Bozeman Science English (auto-generated) English

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hi it's mr. Andersen and this is ap

physics essentials video 14 it's on wave

Hi. It's Mr. Andersen and this is AP Physics essentials video 14. It is on wave-particle

particle duality in the last video we talked about how particles like electrons can have wave-like properties

duality. In the last video we talked about how particles, like electrons, can have wave-like

and in this video I'm going to talk about how things that are normally

properties. And in this video I am going to talk about how things that are normally thought

thought of as waves like light the light that you might see in this laser are

of as waves, like light, the light that you might see in this laser, are going to act

going to act like particles we call those photons and so an object remember can either be a particle like a baseball

like particles. We call those photons. And so an object remember can either be a particle,

or it can be a wave like the sound wave

like a baseball or it can be a wave, like the sound wave that you are listening to right

that you're listening to right now simply vibrating the air and scientists have found this wave particle duality so

now. It is simply vibrating the air. And scientists have found this wave-particle duality. So

they can have properties of both and so light which we normally think of as

they can have properties of both. And so light which we normally think of as being a wave

being a wave has particle-like properties and therefore scientists call it a photon it's a discrete unit of

has particle-like properties. And therefore scientists call it a photon. It is a discrete

light or a quanta of light now what's interesting about light is that it has

unit of light or a quanta of light. Now what is interesting about light is that it has

no mass and travels at the speed of light and so knowing what you know about

no mass and travels at the speed of light. And so knowing what you know about classical

classical mechanics how could we have something that has no mass have energy

mechanics, how could we have something that has no mass have energy. If you think of the

if you think of the kinetic energy equation that's one-half MV squared and

kinetic energy equation, that is one-half mv squared. And since there is no m, how could

since there's no M how could it ever have energy likewise with momentum how could we multiply its mass times

it ever have energy. Likewise with momentum. Hope could we multiply its mass times velocity

velocity to figure out its momentum well scientists like Albert Einstein were

to figure out its momentum. Well scientists like Albert Einstein were able to analyze

able to analyze the results of the photoelectric effect and that they found that the energy is related to the

the results of the photoelectric effect. And they found that the energy is related to the

frequency of the wavelength and the momentum to the wavelength and of course

frequency of the wavelength and the momentum to the wavelength. And of course frequency

frequency and wavelength are tied together as well and so what is the

and wavelength are tied together as well. And so what is the photoelectric effect? Well

photoelectric effect well this had been noticed a long time before Einstein essentially if you shine UV light on

this had been noticed a long time before Einstein. Essentially if you shine UV light on metal

metal it'll start to spark and so the question is how is that energy being

it will start to spark. And so the question in, how is that energy being delivered to

delivered to the metal itself and so Einstein wanted to kind of address the

the metal itself? And so Einstein wanted to kind of address the question, is it a wave

question is it a wave or is it a particle and so if we just represent that light moving from the from the

or is it a particle? And so if we just represent that light moving from the light to the metal

light to the metal as a wave what are some properties of a wave well it's going to have a specific frequency and

as a wave, what are some properties of a wave? Well it is going to have a specific frequency.

therefore a wavelength and so the energy will start to build up on the metal it

And therefore a wavelength. And so the energy will start to build up on the metal. It does

doesn't matter the frequency is if we can increase the amplitude or the size or intensity of

not matter what the frequency is. If we can increase the amplitude or the size or intensity

that wave we should be able to affect that object now if it's acting like a

of that wave we should be able to affect that object. Now if it is acting like a particle,

particle what is that going to be it's simply going to move from the light to the metal now if it's a really light or

what is that going to be? It is simply going to move from the light to the metal. Now if

low-energy particle it won't impact that

it is a really light or low energy particle it will not impact that object and have any

object and have any influence on it but if it's a very large high-energy particle it could eventually even move

influence on it. But if it is a very large high energy particle it could eventually even

that object and so let me show you how the photoelectric effect works what you

move that object. And so let me show you how the photoelectric effect works. What you essentially

essentially have is a light source up here and that's going to be shining on a

have is a light source up here. And that is going to be shining on a piece of metal. Now

piece of metal now that metal is inside a vacuum and if we hit it with the right amount of light it'll generate electrons

that metal is inside a vacuum. And if we hit it with the right amount of light it will

that'll move around the circuit and it

generate electrons that will move around this circuit and it should generate a current.

should generate a current so let's see what happens when we turn that light on you can see that I'm changing the

So let's see what happens when we turn that light on. You can see that I am changing the

wavelength of the light in nanometers but I haven't increased the intensity

wavelength of the light in nanometers. But I have not increase the intensity. Now let's

now let's just choose a red light and we're shining that on the metal and you

just choose a red light. And we are shining that on the metal. And you can see that nothing

can see that nothing has happened none of those electrons are being released and so it looks like the more

is happened. None of those electrons are being released. And so it looks like the more intensity

intensity we add it has no impact on the on the metal itself and so let me decrease the amount of intensity for a

we add it has no impact on the metal itself. And so let me decrease the amount of intensity

second and now let's vary the wavelength let's vary the frequency of that light

for a second. And now let's vary the wavelength. Let's vary the frequency of that light. So

so now we're moving into the yellow and now to the green and then as we start to

now we are moving into the yellow. And now to the green. And then as we start to get

get into the blue and the purple you can see that electrons are being given off

into the blue and the purple you can see that electrons are being given off. And so this

and so this is the first big indication that that light behaves as a particle in

is the first big indication that light behaves as a particle. In other words once we got

other words once we got enough energy in that wave then it started to push off those electrons you can see now as we

enough energy in that wave then it started to push off those electrons. You can see now

increase the intensity we're increasing the current and we could even move it

as we increase the intensity we are increasing the current. And we could even move it into

into the UV light and we're going to get more current over time and so the

the UV light and we are going to get more current over time. And so the photoelectric

photoelectric effect showed Albert Einstein that light is a particle or it acts as a particle what are two pieces

effect showed Albert Einstein that light is a particle or it acts as a particle. What

of evidence once you get the right frequency the electrons are given off

are two pieces of evidence? Once you get the right frequency the electrons are given off

immediately it doesn't have to build up like it would in a wave and then the

immediately. It does not have to build up like it would in a wave. And then the other

other reason is that if we don't have enough energy in that light then we

reason is that if we do not have enough energy in that light then we can not push those electrons.

can't push those electrons are it's not like we can increase the amplitude we literally had to change the frequency to

It is not like we can increase the amplitude. We literally had to change the frequency to

change the energy the light and so did you learn how classical waves display particle-like

change the energy of the light. And so did you learn how classical waves display particle-like

properties and that's how we came up with this whole idea of the photon I

properties? And that is how we cam up with this whole idea of the photon. I hope so.

hope so and I hope that was helpful

And I hope that was helpful.

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