Hi. It’s Mr. Andersen and this AP Physics essentials video 125. It is on photons which
are little packets that make up light. I got an email from somebody and the said they have
a touch light in their house, you can touch it with your finger and it goes through different
cycles, but they could hit it with a UV light and it would cycle through as well. And they
wanted to know what was going on. Well this is the photoelectric effect. And so what you
are doing is you are kicking off electrons with that light. And it was really explained
by Albert Einstein in 1905. Scientists knew that if you hit UV light on metal it would
kick off these little sparks. But he described why that occurred and it also unlocked this
whole idea of light being a particle. And so light is a photon or travels as photons
and those photons can be both waves and particles. And Einstein showed that the amount of energy
in a photon is equal to H. And H is equal to Plank’s constant times the frequency
of the light. So as you increase the frequency you are going to increase the amount of energy
that that photon has. He also showed that photons are quantized. That means that they
travelled in little discrete units. So instead of being along wavelengths stretched out they
are discrete little particles. Evidence came from spectral lines both emission and absorption
spectrum and then the photoelectric effect which we will get into in just a second. And
so remember light is made of up electromagnetic radiation and we are just looking at one small
bit of it. So it goes from really small to long wavelengths and inversely from really
low frequency to high frequency. And so an example of a wave that would have really high
energy, high frequency would be gamma rays. But it is going to have a low wavelength.
And radio waves are going to have long wavelengths but they are going to have really low frequency.
And so our model up to the time of Einstein was that it was a transverse wave. That the
waves are traveling in one direction but there are electric fields and magnetic fields that
act perpendicular to that. But we thought it was one continuous wave that was able to,
for example, interfere. The first piece of evidence that it wasn’t was spectral lines.
And so if you take hydrogen, put it inside a discharge tube and run electrons through
it, it will give off light. But that light is not continuous. What you are going to get
are these discrete lines or these spectral emission lines. And also if you run light
through that hydrogen it is going to absorb the comparable wavelengths of light as well.
And so that suggests that light exists in these discrete little units. Further research
showed really what is going on. And so what is happening is as an electron moves to a
higher energy level it needs to absorb a photon to make that change. And as it falls back
down it emits a photon. And so those photons exist in discrete units. And depending on
what the atoms are you need different amounts of photons to move the energy levels higher
or lower. So this is a model of the photoelectric effect. It is the PHET simulation. I would
encourage you to try it. And so what you can do is you can measure the electrons that are
kicked off the metal in a simple little current. And then you can change the intensity and
you can change the wavelength. Remember if we increase the wavelength we are decreasing
the frequency. And what they found is when you tried infrared light nothing happened.
But as you decreased the wavelength, therefore increasing the frequency, what would happen
is you would get to one point and then all of the sudden you get a flood of these electrons
getting kicked off. And it was not a gradual change. It either kicked off the electrons
and we generated current or it did not. And so what you can use is a graph of that. So
what we are going to do is graph the frequency on the bottom and then we are going to graph
the amount of energy that is created. And so what we find is there is this nice linear
relationship. And so as you are decreasing the frequency you are decreasing the energy.
And then there is no energy at all. And so then as we increase the frequency again what
will happen is we will follow that linear relationship. So this is using copper plates
on either side of this vacuum tube. But we could change it to something else. We could
change the metal, for example, to zinc and now we are going to change the frequency and
we see that same graph, that linear relationship between the frequency and the energy. What
did that show Einstein? Well that showed him that there is a direct relationship. And so
we could figure out how much energy is being released. And so all you do is simply take
the frequency times Planck’s constant and that tells you the energy of an individual
photon. Because it is one photon that is kicking off one electron that is generating the amount
of energy that we have. And so this is our formula. Energy of a photon is Planck’s
constant times the frequency. As we increase the frequency they are increasing the energy.
And so did you learn to support this photon theory using the photoelectric effect? I hope so.
And I hope that was helpful.