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Hello everyone, welcome back to Exploring
Quantum Physics, I'm Charles Clark.

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This week we're going to look at solving
some of the practical problems of

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quantum mechanics, using the vehicle, of

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the theory of atomic structure and
spectra.

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We'll start with an overview of Optical
Spectroscopy.

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It's a subject of great scientific and
technological importance.

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in some sense, it's responsible

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for modern telecommunications
infrastructure, the knowledge

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that we have, about the properties of the
sun and the stars.

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And it's one that provided really
compelling evidence, very

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deep quantitative evidence, for the
existence of quantum mechanics.

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Now, from the course home page, you can
get to this section Additional Materials,

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and within it you'll find a section called
Original Scientific Literature,

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and we will be using two of the papers
here in the homework,

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Bohr model of the atom and the
Photoelectric effect, a paper by Einstein.

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you'll really need to

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look at these, in order to answer some of
the homework questions.

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I mean, it does not require the reading of
the full paper, but I hope you

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will get some enjoyment out, out of having

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read original work, by Niels Bohr and
Albert Einstein.

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[SOUND]

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You're all familiar with the beautiful
rainbow, an

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effect seen when sunlight is scattered in
a certain

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way, and it seems that all the colors

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that we can perceive, are found within the
rainbow.

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Now here's an important tool for
spectroscopy.

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We have white light, entering into this
prism here.

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And then you see output, a, a band of
colors, running from red to blue.

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This technique, this is basically what you
might call a

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deep multiplexing system, the pure white
light comes in and what

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comes out is a mix, is a spread out
mixture of colors.

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And this was a technique used by Isaac
Newton to demonstrate, that white light

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of the sun was indeed a mixture of colors,
because what he did was,

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to take, pick off a particular color,
let's say the blue, and put it

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through a second prism to find that, no
further separation of the color occurred.

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So, from the modern perspective,

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color is a, is a proxy for the, the
wavelength or

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the frequency of the light, which are
related by this characteristic.

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Wave equation, the frequency of a wave
motion,

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is equal to its speed divided by its
wavelength.

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[SOUND]

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Now here's a rather clever demonstration
of

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the inverse action, of the separation of
light.

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This is a, multiplexing effects, where one
combines a a violet,

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a green, and a red light, and the, by the
following

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way, there's a, there's a, these are, you
can't really see

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it, these are cylinders of water, with a
hole bored into it.

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And when

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they're illuminated from behind, the laser
light is entrained

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in the stream of water, that's falling
into a dish.

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And what you see coming out of the dish,
is a scattered light, it's just

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a random mixture of the violet, the green,
and the red colors, so it looks white.

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So in other words, the, this shows that,
the

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combination of these three different
colors, gives apparent white light.

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Now,

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I use this illustration in part, to
introduce the

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three reference lasers, that we'll use for
discussions during the course.

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these are three commonly used laser
lights, that you've seen in practice.

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For example the, the red laser pointer at,
with a wavelength of 650 nano-meters,

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is widely used in, in lectures and

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presentations, as is the green laser
pointer.

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The diode pumps, solid state green laser
pointer, with a wavelength of 532

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nano-meters.
And then the Blu-Ray laser player utilizes

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a violet laser, with a wavelength of 405
nano-meters.

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These spread the range of human vision, as
we'll see

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and so, they provide a good set of basic
tools.

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If you want to remember properties

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of light, it's nice to have some specific
examples, so we'll use

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these, in a number of points in the
lectures and the homework.

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Okay, now we'll just have the first and
only, in-line quiz of this introductory

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lecture, to give you some practice in, in
thinking about the properties of light.

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So the main message of the in-line quiz

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was that, when you combine photons, two
photons to

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produce the third, you add the energies of
the

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two photons to get the energy of the
third.

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So that the, the light from the green
laser pointer, is made by

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combining the energy of two infrared
photons.

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Now the rainbow is a very smooth appearing
object, and those, those colors that

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you saw dispersed by the prism, also seem
to form a relatively smooth band.

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But about almost exactly 200 years ago
Joseph

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Frauenhofer, looking at the spectrum of
sun with

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high resolution, found that it exhibited a
number

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of rather sharp features, of a mysterious
type.

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And here is a modern study of the spectrum
of the sun.

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The rainbow has sort of been, spread out
here.

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And stretched, so that you can see very
narrow regions

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of wavelength, where there are dark spots
in the structure of the sun.

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here are the, here are reference lasers
laid out

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roughly, where their wavelengths lie and
this, this

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notch here, is going to be of great
interest to us later

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on.
So when this phenomenon was discovered it

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was not understood at all and there're no,
there's no physical

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theory that suggested, why the smooth
light of the sun,

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should be disrupted by so many appearing
imperfections.