Hi. It's Mr. Andersen and this is AP Physics essentials video 14. It is on wave-particle
duality. In the last video we talked about how particles, like electrons, can have wave-like
properties. And in this video I am going to talk about how things that are normally thought
of as waves, like light, the light that you might see in this laser, are going to act
like particles. We call those photons. And so an object remember can either be a particle,
like a baseball or it can be a wave, like the sound wave that you are listening to right
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 being a wave
has particle-like properties. And therefore scientists call it a photon. It is a discrete
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 classical
mechanics, how could we have something that has no mass have energy. If you think of the
kinetic energy equation, that is one-half mv squared. And since there is no m, how could
it ever have energy. Likewise with momentum. Hope could we multiply its mass times velocity
to figure out its momentum. Well scientists like Albert Einstein were able to analyze
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
and wavelength are tied together as well. And so what is the photoelectric effect? Well
this had been noticed a long time before Einstein. Essentially if you shine UV light on metal
it will start to spark. And so the question in, how is that energy being delivered to
the metal itself? And so Einstein wanted to kind of address the question, is it a wave
or is it a particle? And so if we just represent that light moving from the light to the metal
as a wave, what are some properties of a wave? Well it is going to have a specific frequency.
And therefore a wavelength. And so the energy will start to build up on the metal. It does
not matter what the frequency is. If we can increase the amplitude or the size or intensity
of that wave we should be able to affect that object. Now if it is acting like a particle,
what is that going to be? It is simply going to move from the light to the metal. Now if
it is a really light or low energy particle it will not impact that object and have any
influence on it. But if it is a very large high energy particle it could eventually even
move that object. And so let me show you how the photoelectric effect works. What you essentially
have is a light source up here. And that is going to be shining on a piece of metal. Now
that metal is inside a vacuum. And if we hit it with the right amount of light it will
generate electrons that will move around this circuit and it should generate a current.
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 have not increase the intensity. Now let's
just choose a red light. And we are shining that on the metal. And you can see that nothing
is happened. None of those electrons are being released. And so it looks like the more intensity
we add it has no impact on the metal itself. And so let me decrease the amount of intensity
for a second. And now let's vary the wavelength. Let's vary the frequency of that light. So
now we are moving into the yellow. And now to the green. And then as we start to get
into the blue and the purple you can see that electrons are being given off. And so this
is the first big indication that light behaves as a particle. In other words once we got
enough energy in that wave then it started to push off those electrons. You can see now
as we increase the intensity we are increasing the current. And we could even move it into
the UV light and we are going to get more current over time. And so the photoelectric
effect showed Albert Einstein that light is a particle or it acts as a particle. What
are two pieces of evidence? Once you get the right frequency the electrons are given off
immediately. It does not have to build up like it would in a wave. And then the other
reason is that if we do not have enough energy in that light then we can not push those electrons.
It is not like we can increase the amplitude. We literally had to change the frequency to
change the energy of the light. And so did you learn how classical waves display particle-like
properties? And that is how we cam up with this whole idea of the photon. I hope so.
And I hope that was helpful.