Well the quad electric effect clearly suggest that, that the light sometimes can
behave as a beam of particles, the other players in the effect at the electrons
will not really suspect it to represent anything but particle like entities.
However this lecture picture was two questions by number of theories back in
the beginning of the last century. Another by Prince Louis de Broglie, who
suggest that massive particles can sometimes behave as waves.
He was also bit often thinking that the photon also has a mass, but from apart
from this idea that didn't quite work out his other hypothesis turned out to be
correct. But it was actually serendipity that plays
such an important role in Physics sometimes, that produce the clearest
experimental results that confirmed de Broglie's hypothesis and put a nail in the
coffin of classical Physics. So let me tell you about his experiment
and actually a rather curious story behind it.
In April of 1925, these two gentlemen Clinton Davisson and Lester Germer, were
working in the physics lab at the Bell Laboratories and they were studying how
electrons skitter off of a nuclear target. They probably didn't expect any
groundbreaking results when they started this experiment and their initial results,
as a matter of fact, were consistent with their sort of boring expectation of a
diffusive scattering of electrons off of the target.
So here's the diagram of the paper which shows the from the original paper they
eventually published in 1927 which shows the apparatus they used.
So here we have the, this part which is a gun, electron gun which shoots electron,
electrons towards the target and so this part is a nickel target they were
studying. So another part of the apparatus is this
dectector which detects electrons and they can move it around and by doing so they
can study the angular distribution of the scattered electrons.
However, what this diagram on the paper don't discuss and don't show is the full
history of the event. Namely, the diagram doesn't show a bottle
of liquid air that was sitting nearby and which exploded due to the heat coming from
this target presumably. And so, this error and this smashed the
vacuum chambers surrounding this apparatus and the air rushed into the chamber,
oxiding him and oxidizing the nickel target.
So this was unpleasant by itself, perhaps but it also made the target they were
using completely useless for the experiments, because it was completely
oxidized. So Davisson and Germer decided to save the
target by heating it up and by nailing it to get rid of the oxide.
So, they did so and put it back into the chamber.
And so, they repeated their experiment the new target, well the old target but which
was should be nailed. But to their amazement, what they saw the
picture of scattered electrons that they saw was completely different from the
picture they had seen before the accident. But why did the results change?
What was different? In trying, in trying to resolve this
mystery data sent in Germer examine the nickel target with a little closer.
And concluded that heating it up and nailing it resulted in this transformation
from a fully crystalline form which basically implies a number of small, small
crystals put together in random fashion. To a few single crystals with a perfect
crystalline order inside. So and these eventually explained the new
results. But what are those new results?
So instead of random blob of scattered electrons, they now saw clear peaks
corresponding to the electrons beams reflected from the crystal at certain
specific angles. And this [inaudible] were strongly
dependent on the energy of incoming electrics.
So, this lot here is actually the plot from the original paper by Davisson and
Gerner, and it shows circles scattering curves for what they call the 54 wall and
65 wall electron beam. That was not once referred to the energy
of electrons. Perhaps a more illuminating modern picture
of the same phenomenon they discovered is of course, a different experiment but
maybe different material, but the same phenomenon, now known as electron
diffraction is presented here. And so what is remarkable here is that
this found phenomenon is something that only waves can exhibit.
Upon propagation through irregular ray of scattering.
So these bright spots here correspond to the directions along which the intensity
of the scattered electron beam is high and so this phenomenon is known.
So here is a reminder or of this picture so if we sent away towards the crystal,
depending on the handle the reflective waves from different layers they can
either enhance each other if they appear in phase or they can appear out of phase
in which case they will cancel each other out, and this will correspond sort of dark
regions in this block. And well, this had been known before
Davisson and Germer's experiment, and they readily recognized it, and found that
their data made perfect sense if the liberal hypothesis were to be accepted.
And so, they actually, say in their paper, explicitly [inaudible] here's a quote from
the paper, the most striking characteristic of these beams Is a one to
one correspondence, which the strongest of them bear to the beams that would be found
issuing from the same crystal if the incident beam were a beam of x-rays.
So they're saying if they were to scatter x-rays over the same crystals, they would
show the same picture. And finally, they say this so their data
made sense, if, if it involves association of a wavelength.
With the incident electron beam and this wave length turns out to be in acceptable
driven with the value of h over nb, h being the blanks actions constant divided
by the momentum of the electron. So this was a rather remarkable
verification of Prince de Broglie idea and the main equation, which again is this
equation, which relates. The properties of a particle with a mass
moving with a certain velocity to a wave that would be associated with this with
this particle. And these ideas brought him a Nobel Prize
just for years later. So to summarize, let me just tell you what
we've learned so far in the last two segments.
So far in the first lecture, we have talked about two Nobel Prize winning works
the photoelectric effect and the electron diffraction, and all in all we met in
passing at least at least six Nobel laureates in our first lecture Feynman,
Michelson, Lenard, Einstein, Davisson, de Broglie, so not too bad.
And we have seen, and these are basically the main sort of conclusions from the two
experiments we just discussed. We have seen that there is a clear
experimental evidence that light behaves sometimes as a beam of particles carrying
energy quanta. And the frequency relates to the energy as
so, where the wavelength of light relates to the energy according to this expression
where the coefficient of proportionality is the Planck constant.
So here, I mean, I see the frequency of light C is the speed of light and lambda
is the wavelength. So on the other hand there is also
evidence that the electrons message particles, sometimes behave as waves.
And the relation we just discussed with these liberal relations.
And in the next lecture, we'll try to figure out what is going on here.