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VIVIAN SONG: In
this video, we will

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be going over a Goodie
Bag number two which is

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about electronic transitions.

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What you'll need are four LEDs--

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white, blue, green, and red--

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and one spectrometer.

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The objectives in this video are
to understand photon absorption

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and emission, apply
the Bohr Model,

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and use the spectrometer
to see sample spectra.

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The conceptual questions
you should keep in mind

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are, how do electrons transition
to different energy levels?

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And why is spectroscopy a method
of material characterization?

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As a reminder,
the Bohr Model can

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be used to model these
electronic transitions,

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assuming that there's only
one electron in the atom.

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So right now, I'm going to
draw my energy axis and three

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different energy levels that
my electron can hop into.

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So this is one,
and two, and three.

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So let's say that our electron
starts in the ground state.

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And it absorbs
some energy and can

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hop to a higher energy level.

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But in doing so, at this
higher energy level,

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it is in a more unstable state.

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Eventually, it'll want to return
to that more stable state.

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And when it does so, it emits
energy in the form of a photon.

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So emit photon
and absorb energy.

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These different
electronic transitions

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can be tracked
with this equation.

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So the change in energy is equal
to minus 13.6z squared times 1

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over nf squared minus
1 over ni squared.

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Where z is the
atomic number, and f

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is the final state
of the electron.

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And i is the initial
state of the electron.

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And this change in energy
is in electron force.

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So remember that these
electronic transitions

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are quantized since these energy
levels are not continuous,

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but they're integers.

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This means that the photon
that the electron emits

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is going to have a certain
set of different wavelengths.

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And we can see that with
this equation energy equals

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hc over lambda, where energy is
equal to this change in energy.

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h is Planck's constant.

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c is the speed of light.

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And lambda is the wavelength
of our emitted photon.

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After looking at the
red and white LEDs

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through the spectrometer, you
should see something like this.

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Notice how for the red LED,
mostly the red and orange bands

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are visible.

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Whereas for the white
LED, almost all the colors

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are visible.

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Another thing that
you could explore

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is using your spectrometer
to look at the ceiling lights

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and seeing how that spectra
would differ from the spectra

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that you observed
from your LEDs.

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In summary, electrons
absorb and emit

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photons of different wavelengths
because electronic transitions

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are quantized.

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By capturing this
information, the spectrometer

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becomes a very useful tool for
characterizing our materials.