Hello everyone, welcome back to Exploring
Quantum Physics, I'm Charles Clark.
This week we're going to look at solving
some of the practical problems of
quantum mechanics, using the vehicle, of
the theory of atomic structure and
spectra.
We'll start with an overview of Optical
Spectroscopy.
It's a subject of great scientific and
technological importance.
in some sense, it's responsible
for modern telecommunications
infrastructure, the knowledge
that we have, about the properties of the
sun and the stars.
And it's one that provided really
compelling evidence, very
deep quantitative evidence, for the
existence of quantum mechanics.
Now, from the course home page, you can
get to this section Additional Materials,
and within it you'll find a section called
Original Scientific Literature,
and we will be using two of the papers
here in the homework,
Bohr model of the atom and the
Photoelectric effect, a paper by Einstein.
you'll really need to
look at these, in order to answer some of
the homework questions.
I mean, it does not require the reading of
the full paper, but I hope you
will get some enjoyment out, out of having
read original work, by Niels Bohr and
Albert Einstein.
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You're all familiar with the beautiful
rainbow, an
effect seen when sunlight is scattered in
a certain
way, and it seems that all the colors
that we can perceive, are found within the
rainbow.
Now here's an important tool for
spectroscopy.
We have white light, entering into this
prism here.
And then you see output, a, a band of
colors, running from red to blue.
This technique, this is basically what you
might call a
deep multiplexing system, the pure white
light comes in and what
comes out is a mix, is a spread out
mixture of colors.
And this was a technique used by Isaac
Newton to demonstrate, that white light
of the sun was indeed a mixture of colors,
because what he did was,
to take, pick off a particular color,
let's say the blue, and put it
through a second prism to find that, no
further separation of the color occurred.
So, from the modern perspective,
color is a, is a proxy for the, the
wavelength or
the frequency of the light, which are
related by this characteristic.
Wave equation, the frequency of a wave
motion,
is equal to its speed divided by its
wavelength.
[SOUND]
Now here's a rather clever demonstration
of
the inverse action, of the separation of
light.
This is a, multiplexing effects, where one
combines a a violet,
a green, and a red light, and the, by the
following
way, there's a, there's a, these are, you
can't really see
it, these are cylinders of water, with a
hole bored into it.
And when
they're illuminated from behind, the laser
light is entrained
in the stream of water, that's falling
into a dish.
And what you see coming out of the dish,
is a scattered light, it's just
a random mixture of the violet, the green,
and the red colors, so it looks white.
So in other words, the, this shows that,
the
combination of these three different
colors, gives apparent white light.
Now,
I use this illustration in part, to
introduce the
three reference lasers, that we'll use for
discussions during the course.
these are three commonly used laser
lights, that you've seen in practice.
For example the, the red laser pointer at,
with a wavelength of 650 nano-meters,
is widely used in, in lectures and
presentations, as is the green laser
pointer.
The diode pumps, solid state green laser
pointer, with a wavelength of 532
nano-meters.
And then the Blu-Ray laser player utilizes
a violet laser, with a wavelength of 405
nano-meters.
These spread the range of human vision, as
we'll see
and so, they provide a good set of basic
tools.
If you want to remember properties
of light, it's nice to have some specific
examples, so we'll use
these, in a number of points in the
lectures and the homework.
Okay, now we'll just have the first and
only, in-line quiz of this introductory
lecture, to give you some practice in, in
thinking about the properties of light.
So the main message of the in-line quiz
was that, when you combine photons, two
photons to
produce the third, you add the energies of
the
two photons to get the energy of the
third.
So that the, the light from the green
laser pointer, is made by
combining the energy of two infrared
photons.
Now the rainbow is a very smooth appearing
object, and those, those colors that
you saw dispersed by the prism, also seem
to form a relatively smooth band.
But about almost exactly 200 years ago
Joseph
Frauenhofer, looking at the spectrum of
sun with
high resolution, found that it exhibited a
number
of rather sharp features, of a mysterious
type.
And here is a modern study of the spectrum
of the sun.
The rainbow has sort of been, spread out
here.
And stretched, so that you can see very
narrow regions
of wavelength, where there are dark spots
in the structure of the sun.
here are the, here are reference lasers
laid out
roughly, where their wavelengths lie and
this, this
notch here, is going to be of great
interest to us later
on.
So when this phenomenon was discovered it
was not understood at all and there're no,
there's no physical
theory that suggested, why the smooth
light of the sun,
should be disrupted by so many appearing
imperfections.