Hello everyone. I'm Charles Clark.
And I'm going to conduct some lectures on practical use of quantum mechanics to
solve some real physics problems. The very interesting type.
These are some of the most important problems in the early days of quantum
physics, which led to the understanding of the structure of atoms.
In enough detail to make possible number of applications that are very important.
For example the development of the laser and it's a subject that was not imp-,
important not only in the early days but it's a very vibrant field of research
today. Since 1997, about 11 Nobel prizes in
physics have been awarded. For innovative use of the interaction of
light with atoms to do things like con, create ultra controlled ultra cold
matter. develop new optical frequency standards,
and the like. So it's a very vibrant field of research.
And I hope these lectures will give you some sense of the excitement.
And ma, and an impression of how you could actually approach it yourself.
Now this is a fairly field with a lot of material in it, and I've only time to
give you a brief introduction. So, let, let me just draw to your
attention the fact that we have a page accessible along the main page of the
website, Additional Materials. And we've put in a selection of original
scientific literature with commentary there on various aspects that are
important. So the things that are going to be talked
about in detail in the first set of lectures by me are the Bore model of the
atom. Not necessarily in this order the
discovery of deuterium, fascinating story of which there's both a detailed and a
sort of a simplified account. Recommend you look at.
the, the green laser pointer and the photoelectric effect.
Well, come to think of it, and the Young's Double Slit experiment.
So, there's a very, couple introductory paragraphs in each one of these.
if you find something that interests you in the lecture, I suggest you take a
look. In fact the homework will require that
you actually, read part of Einstein's original paper, on the photoelectric
effect. It's, it's an awesome paper.
And you have a choice of reading it in the, German original, which we provide
you, or in the English translation that is available we give you a link to the
English translation. At Wikimedia.
Okay, let's begin talking about the world.
Here's a happy scene. I'm sure you've all felt like this at
some time, you're walking along and a very clear day, all of a sudden you see a
beautiful rainbow. Now the world is full of colors.
And the one's first impression is that the sun is a sort of a pure white light.
It's a very natural color. It seems to display things.
In some optimal way for us. They often like to inspect an object by
taking it out in the open sunlight and seeing what it looks like.
So the original impression of humanity is that the sun was the source of all light.
Indeed, you know, maybe fires would produce something, but for a long time
there were no artificial lights. And occasionally it would show, these
effects where you would see in the otherwise clear sky a spectrum that
seemed to contain all the possible colors of human experience.
Now here's another example using a fairly natural material, a piece of glass, where
we see a case where a white light comes in and then a number of other colors, red
blue, are emitted. Basically, the white light is spread,
somehow changed into a spread out system of colors.
Now, this is a phenomenon that was known in antiquity.
And people then seemed to think that there was something about the.
The shape of the prism or the qualities of the glass that impressed different
colors on sunlight, which was otherwise a clear white light.
But Newton showed that this was not the case, and that in fact that sunlight was
a mixture of colors. And it's by one of these experiments that
when you hear about it you think it's so obvious.
What he did was, he took sunlight, and put it through a prism.
But then he, he took some of the separated light, a light of a pure color,
and put it into a second prism, and he saw there that no further separation of
the colors occurred. So the natural way of interpreting that
result, is that white light is a, is for some reason a mixture of all these colors
put together. And what the prism does, is separate them
out. we call this the dispersion of the
colors. by the by the prism.
Now, from today's stand point we know that color is an index of the light's
wavelength. Lambda.
Conventional designation for the wavelength of light.
And, there's a famous equation, which you're going to get some more practice
with. The relationship between frequency, Nu,
and wavelength Lambda. So, Nu is equal to c over Lambda, where C
is the speed of light. Oh, so C has the units of meters per
second, or length divided by time, Lambda has the units of length.
And what is a wavelength, a wavelength is a wave motion.
The wave length is the distance over which the wave motion begins to repeat
itself and C is the speed of light. So what we as you're going to see there
is a conventional way of designating the characteristics of light in the optical
region of the spectrum and I'll explain to you why that is.
Now, here's a little inverse example of what you saw in the prism.
A very elegant and interesting demonstration made by Alexander Albrecht
of the University of New Mexico. This is a prize winning photograph from
the membership magazine of the Optical Society of America, December 2012.
So, what is done here is, there are these three three pipes of water, there's water
streaming into a bowl and then a laser is put into the into the pipe and the laser
beam is entrained in the water. This is a demonstration you may have seen
before, but what's done here is a red, green and a violet laser are used to, put
the light of separate colors in the bowl. Then the light scatters around, and so
what you see coming out from your eye is, it's a, it's a scattered combination
containing, you know, more or less the same quantity of red, green and violet.
So that gives a white appearance, very nice.
Now the these three lasers are of a common type and we're going to, they're
all actually available as laser pointers. And, and, they light in this region of
wavelengths is available as a very inexpensive laser pointer, costing
between five and ten American dollars. And so we are going to use, throughout
these lectures, these three lasers as sort of standard reference lasers, for
the for the discussion of the course. So the red laser, at a wavelength of 650
nanometers, a green laser, at a wavelength of 532 nanometers, this is the
famous diode pump solid state. The wavelength produced by diode pump
solid state architecture that's used in the green laser pointer.
And then finally, the wavelength of four or five nanometers, which is the
wavelength. Sorry, wavelength of the blue, using the
blu-ray laser, so if you have a blu-ray system or a Playstation at home, you are
the already the proud owner of one of these four or five nanometer lasers.
Okay, and so now as an introduction to. The application of some of these basic
concepts of optics. I mean, I'll give you a little inline
quiz, that has to do, with the properties of one of my favorites, which is the the
green laser pointer. Okay, I hope that some of you found that,
example, relatively easy. I hope all of you found it at least
interesting. The idea that you can use a passive
crystal to change the color of light is something that's only become possible
well since the development of the laser in the 1960's.
And to learn a little bit more about that I recommend this paper on the green laser
pointer it's written, I hope in a fairly accessible way it's not highly technical
and does provide a simplified description of the operation device if you're
interested. Now one thing that you're going to have
to do if you want to be a successful physicist or a successful scientist any
time, is to lose your sense of complacency.
When you wake up everyday, you see the white light of the sun.
You see the rainbow from time to time, you see the same colors over and over.
You might think, what else is there? Well, the answer is revealed.
By being able to look where no one has looked before.
And that's really the, why we do quantum physics today, is to try to understand
things at a deeper level. And here is an example of how that type
of deep understanding, made on an observation by a man who, you know, alone
in the world at the time, had the capability.
Of making those detailed observations made a tremendous change in science and
history, and that man was Joseph von Fraunhofer, one of the most skilled
optical physicists of his day. He was a specialist in making various
types of optical glass that had very high dispersion, so he could make prisms.
That would separate out light to greater the different colors of light to a
greater degree than any one else, and here is a paper from his publication and
in fact that you can acquire that publication through our additional
materials uh,document of course its written in German actually rather an old
fashioned type of German. Maybe some students in the German
discussion group can comment upon that. But this is a drawing produced by
Fraunhofer of what he saw when he looked at the light of the sun using a highly
dispersive prism. He saw dark lines in the spectrum.
So, imbedded in this rainbow, there were patches of darkness.
Hundreds of them. Now, well, might you ask, how did he know
that those were patches in the light of the sun itself, rather than maybe
something in the earth's air, that would be blocking?
would, would be doing something in the sunlight.
Well it's a good question, and Fraunhofer answered it, at least in part.
By also looking at the light from other stars, in particular the star Sirius, is
one that he reports. And he saw other dark lines there, but
not necessarily exactly the same dark lines that were in the sun.
So, in other words, at least a number of these dark lines are definitely
associated with the light of the sun. Now, you know there's been many ideas of
color and of the nature of the sun and everything throughout history.
But I don't know of a case where someone's just sat down and said, well, I
bet there's a whole bunch of dark lines in the light of the sun that we just
haven't seen yet. And maybe someday, someone's going to
invent some optical device that will allow them to be revealed.
So now let's take another, we'll take a look.
At, a more modern observation, the same thing.
Behold the crown of the great king of the solar system, the sun.
Here's a wonderful image, that's a spectrum of the sun seen at high
resolution produced at the, the, National Optical Astronomical Observatory in
Tucson, Arizona. And let me just describe to you how this
was taken. Let's say that you're looking at the
rainbow in the skies. So here's the red bow that's above and
then down at the bottom, there's a violet one, and then, from violet, we go to
blue, I guess it is. Then to green.
And to yellow. And, and orange, and so on back up to the
red. So here's a, here's a crude drawing of
the rainbow. So you, if you, if you had a picture of
that. And then you, cut out, a little thin
strip like that, now you take, this thin strip.
Orient it here. So for example this could be the red, the
sort of, the red patch. And then down here this would be the
violet. Then what you do is that you, you cut out
50 of these strips, like this and then what you do is you stretch.
Stretch each one of these out to expand the region so this enables you, in other
words if you were to take these 50 strips there 50 strips stacked vertically here.
And if you lay them end to end you would see the rainbow but with the resolution
that allows you to see all this fine detail.
Now for your guidance I have positioned some of our reference lasers near where
they would be in this spectrum. Here's 650 nanometers, 532.
The blu-ray laser. And now what this indicates is that, the
wavelength is I'll, I'll draw it on this side.
The wavelength is increasing within a strip as you go from the right to the
left because this is short wavelength, and you're going backwards, you see.
So you're, you're increasing the wavelength.
And then it's also increasing as you go from one strip to the next.
Is that clear? Well you're going to now get an
opportunity to see whether you grasp the concept.
Okay again I hope that some of you got the, material on the online quiz and, and
everyone found at least. Interesting enough to, to think about the
problem that's not it's not complicated. A heavy map but its a good experience in
actually you know looking at the results of experimental data and understanding
very clearly what that representation means because these things aren't always
obvious. Okay, that's it for today, and I will,
well, that's it for this lecture. We'll resume and talk more about
interference, and attraction.