Hi. It’s Mr. Andersen and this is AP Physics
essentials video 86. It is on emission and absorption spectra. If we were to take a gas,
like hydrogen, and put it in a discharge chamber and shoot electrons at it, it is going to
give off light that looks like this. If we were to analyze that light, if we were to
split it in a prism, it would look like this. So instead of being a rainbow of light we
are going to get these discrete units of light or these discrete protons that are carrying
discrete amounts of energy. And this was puzzling to scientists for a long time. We could also
shine light through that discharge chamber and parts of it would be absorbed. And so
this would be the absorption spectra. So most of the light would make it through but some
of it is not going to make it through. And this puzzled scientists for a long time until
Niels Bohr finally figured it out. And what is happening, if we look at a hydrogen just
in the first two energy levels, is that as the electron goes around the atom, as it jumps
to a lower energy level, it is going to give off a photon of light. And for it to move
up to that other energy or that higher energy level it is going to have to absorb a photon.
And so there are discrete colors of light that are required for it to jump up. And as
it jumps down it is going to release these discrete colors of light as well. If you hit
it with other colors it is not going to do anything. It does not have the right amount
of energy. And so this is conservation of energy. Conservation of energy in an atom
or in a nuclei, when it absorbs a photon, so we call that absorption. And so what does
that mean? The amount of energy in the photon and the atom or nuclei before absorption is
equal to the amount of energy inside the atom or nuclei after absorption. And same thing
applies to emission. And so as we give off that photon energy is conserved. The amount
of energy we had in the atom or the nuclei before is equal to both the photon and the
atom or nuclei after. Now this is really valuable in science because we could look at what is
being given off, so we can look at the spectra of emissions from any kind of an element or
molecule, and it tells us what elements are going to be found inside it, if we know what
those energy levels are. And so if you know anything about visible light, it is all the
colors of the spectrum. So as we shine it in a prism, as it moves through the glass
different wavelengths of light are going to travel at different speeds and so it is going
to split it into the spectrum. Now know this, on the red side there is a part of the spectrum
that we cannot see. That is infrared light. And there is going to be UV light on the other
side. And we are just seeing that visible spectrum that is going to be right in the
middle. And so if we look at those energy level diagrams, for example in a hydrogen
atom, if we were to hit it with the right color of photons, so let’s shine on it the
right color photon, let’s say a red photon, watch what happens when it hits that electron,
it jumps to a higher energy level. Now let’s say it falls down to a lower energy level,
what kind of color is going to be given off from that electron? It is going to be that
red photon again. But what happens if we hit it with a green photon for example, and it
does not have the right color? It does not have the right energy? It is never going to
be able to move. Let’s say we hit it with a yellow photon. What does it do? Again nothing.
And so these atoms are being bombarded by different colors of photons but it has to
be the right color. Let’s say it is the right purple color, what is going to happen?
It is going to jump to a higher energy level. What happens as it falls down to that ground
state? It is going to give off that same exact photon. So you can model this. This is a PHET
simulation. What I am going to do is take one hydrogen atom, put it in a discharge chamber.
I am going to use a cathode ray to hit it with an electron. And watch what happens.
It gets high energy and as it falls back down it gives off a certain color of photon. Now
if I were to hit it with not just one electron, but a continuous stream of electrons, before
it is able to fall down again it is giving off different colors of photons. So we are
getting all of these different colors of photons and you can see on the spectrum on the bottom
of this page it is kind of plotting what color those photons are. So now let’s do multiple
atoms. Let’s do a bunch of atoms. We are going to hit those with a continuous stream.
And so what is happening is they are all at different energy levels and so if we were
to plot the average of all of those photons that are being given off that is going to
be that purple color we get. But if you were to do a spectrogram on it you could see on
the bottom that we are getting those bars. Now way to the right we have the infrared.
Way to the left is the UV. But you can see those four spectral bars that I just talked
about a second ago. Now let’s say it is not hydrogen in the middle. So this is going
to be a hydrogen spectrum here. I have sped up the simulation. So we are getting those
bars. But let’s say we change it to mercury. What are we going to get? Totally different
electron. Totally different energy levels. And so we are going to get different color.
This is what sodium would look like. This is what neon would look like. And so when
you look at a neon light and it gives that reddish color it is the sum of all those different
photons that are being given off from that neon gas. And so did you learn to describe
emission, again that is what is given off, and absorption spectra? And associate that
with electron or nuclear transitions? I hope so. And I hope that was helpful.