Emission and Absorption Spectrum

by Jared Rovny

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    00:01 The emitted photons will have an energy that depends on the difference in the energy levels from where it starts to where it jumps to.

    00:10 This means that any electron that's jumping energy levels will emit a photon with a different amount of energy, a different frequency of the light.

    00:19 So in the diagram you can see here.

    00:21 For an electron that jumps a small distance, may be between two very close energy levels, it's not going to emit much energy.

    00:28 So it's going to emit those lower frequencies or lower energy photons of light.

    00:33 And those we already discussed are the more red kinds of light.

    00:36 For big jumping energy, say between the second and the first energy level, there's a big difference there.

    00:42 There's a big difference between the ground state and that first excited state.

    00:46 And so when the electron jumps between a bigger gap like that, it's releasing a lot more energy, which means that the photon that it releases will also have this high energy.

    00:54 And as we discussed those high energy photons are more towards the blue and purple higher frequency parts of the spectrum.

    01:02 This gives us a very interesting way to look into the atomic structure of materials that we can't necessarily look into by eye.

    01:08 This is called an "Emission Spectrum." When we look at the light that is coming from something and separated into the different frequencies of light.

    01:16 So again we mentioned that for an atom as the electrons jump the energy levels, the amount of energy, the color of the photon they emit will depend on the spacing, the energy difference between the two bends that the electron jumps to.

    01:30 So again if we took the light that's coming from some atom like a hydrogen gas, we could separate the light by putting it through something like a prism.

    01:38 And we could see the different components of light by casting it's shadow if you will on a sort of bent where we can see the different colors of light hitting the wall.

    01:47 We call this band of different colors that comes from some atom like hydrogen gas, the emission spectrum.

    01:54 Because we see a spectrum of different colors and they are emitted from some sort of atom or molecules or some sort of a gas.

    02:02 So this emitted spectrum is very interesting to us because it acts as a source of finger print of a different source of atoms.

    02:08 Because these energy levels that we discussed.

    02:11 All these different bands that you can have are unique to different atoms.

    02:15 So for example the hydrogen atom has a very particular shape to it's orbitals, the different energy levels.

    02:21 Or a different sort of atom may be nitrogen gas or oxygen or something like this.

    02:25 We'll have a very different shape to it's energy levels.

    02:28 This means that as electrons jump around between the energy levels as we described, each different kind of atom or different kind of a gaseous state will emit different colors of light.

    02:39 And again we can look at those colors just using something like a spectrum that we have here.

    02:44 So suppose we have light for example coming from a distance star, we could still do the same thing.

    02:49 We could put a prism.

    02:50 See the light coming in.

    02:51 Separate the components of the light and get a band structure just like you see in the emission spectrum here.

    02:56 And determine what colors of light were coming even from something like a distant star or really any other object as well.

    03:04 There's one other thing we could do which is exactly the opposite of this.

    03:08 Rather than having the electrons fall in their energy levels and emit light.

    03:12 We could instead send light in to may be to a gaseous state like sending light through hydrogen gas.

    03:18 And then whichever energy levels of that light whichever photons like the red photons or the purple photons are consistent with the energy differences in the bands will be observed by the electrons.

    03:30 So far example, look at the just first jump here between the ground state and the first excited state for the electron in this atom below.

    03:37 If we sent in a photon with exactly the right energy like a purple photon, the electron would have absorbed that energy and jumped between energy states.

    03:47 States going from the ground state up to the first excited state.

    03:51 The interesting thing for us is that if we send that light through and the electrons take that particular frequency of energy, then when we repeat the same sort of thing and cast the light on to a spectrum like this.

    04:02 We get a series of shadows or dark spots in our band because our light is no longer there.

    04:06 It's no longer there because it was taken by the gas like the hydrogen gas as we sent light through it.

    04:12 So this is exactly the same as in emission spectrum.

    04:14 But in fact we call this the opposite.

    04:16 We say this is an absorption spectrum because the gas that we were sending the light through is absorbing the energy that we've sent through that gas.

    04:26 So this wraps up our summary of the first introduction to the electronic structure of atoms.

    04:31 We're going to build on these ideas that we just introduced specially with the hydrogen atom and go further and explore some more intricate shapes having to do with the kinds of orbitals that electrons can be in.

    04:41 But that's next time.

    04:43 Until then, thanks for listening.

    About the Lecture

    The lecture Emission and Absorption Spectrum by Jared Rovny is from the course Electronic Structure.

    Included Quiz Questions

    1. It releases a more “red” (long wavelength) photon.
    2. It releases a more “blue” (short wavelength) photon.
    3. It always releases the same type of photon because all energy levels are discretely spaced.
    4. It releases a more “red” (short wavelength) photon.
    5. It releases a more “blue” (long wavelength) photon.
    1. The distribution of electron energy levels
    2. The distribution of neutrons relative to protons
    3. The distribution of photons released by the nucleus
    4. The distribution of energy among the particles in the nucleus
    5. The distribution of colors of electrons
    1. Absorption spectra reveal missing wavelengths where the light was absorbed.
    2. Absorption spectra come from the release of photons from the gas.
    3. Absorption spectra are brighter at the emitted wavelengths.
    4. Absorption spectra are bright at twice the frequency.
    5. Absorption and emission spectra will always have identical appearances.

    Author of lecture Emission and Absorption Spectrum

     Jared Rovny

    Jared Rovny

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