Professor Dave again, let's continue our
discussion of quantization.
We are now moving through the initial
discoveries that brought about modern
physics. Planck set things into motion in
1901, and the man who was to carry the
torch next was none other than Albert
Einstein. In 1905, while working as a
patent clerk in Switzerland, he published
three seminal papers that revolutionized
physics. These were about Brownian motion,
special relativity, which we will get to
later, and the photoelectric effect.
The last of these, just like Planck's work, also
made use of quantization, which cemented
the concept as more than just a fluke.
The photoelectric effect has to do with
the way light is able to eject an
electron from a piece of metal. Given
that this subject matter is also of
great concern to chemistry, we have
already covered this topic in the
general chemistry course. I highly
recommend clicking on the card you see
now to view this in-depth analysis of
the photoelectric effect, as it is a
crucial step in modern physics. If you
have familiarized yourself with this
concept, we will simply recall that
Einstein's work showed that only light
above a certain frequency could eject an
electron, regardless of the intensity of
the beam, and for this reason he proposed
that light was comprised of individual
quanta called photons, whereby it was an
individual photon of sufficient energy
equal to h times f that was able to
eject an electron. This meant a number of
things. First, it wasn't just the
vibrational energy of the atoms in the
blackbody that was quantized. Light is
also quantized, since photons are quanta.
So quantization seems to be here to stay.
But furthermore, since it had already
been well established that certain light
related phenomena like diffraction and
interference patterns are best explained
using a wave model, it must be the case
that light can be described
as both a particle and a wave. This
bizarre concept is called wave-particle
duality, and though it is essentially
impossible to visualize how something
can be both a particle and a wave, this
is the kind of quantum weirdness we are
going to have to get used to if we are
to learn modern physics. In order to
grasp the material moving forward, we
must let go of our sense experience and
its suggestions of what waves and
particles must be, things like ocean
waves and baseballs, and realize that in
the realm of the submicroscopic these
notions simply do not apply. Moving on
from Einstein, Niels Bohr showed that
quantization of energy also applies to
the energy of an electron in a hydrogen
atom. Bohr proposed that the electron can
only inhabit specific energy levels, and
that it will move between these energy
levels when absorbing or emitting a
photon of an energy that is equivalent
to the difference in energy between the
two energy levels involved in the
transition. This model was able to
explain the emission spectrum of
hydrogen and other elements, and by
extension, the color of every object that
reflects light. More information on the
Bohr model can be found in the general
chemistry series, or by clicking on the
card you see right now. After this, de
Broglie demonstrated that it is not
just light that exhibits wave-particle
duality, but particles of matter as well.
This meant that the electron, just like
any other particle, has a wavelength that
depends on its momentum, which
complicated matters for chemistry quite
a bit. This notion was soon corroborated
when a beam of electrons was shown to
exhibit a diffraction pattern, just like
a beam of light does. This meant there
was no turning back. Waves can act like
particles and particles can act like
waves, whether we are talking about light
or matter. Because Newtonian mechanics is
unable to fully describe this kind of
behavior, we
had to develop an entirely new field to
do so, and that field is called quantum
mechanics. This and the figures that
contributed to its development will be
the focus of the next few tutorials.
Once again, tutorials 12 through 14 from the
general chemistry course cover the
photoelectric effect, the Bohr model of
the hydrogen atom, wave-particle duality,
and some basic quantum terminology in
greater detail than we have mentioned
here, so it is highly suggested that you
take a moment to review these materials
before proceeding with the modern
physics course. If you are good to go,
let's move on to some new concepts.
Thanks for watching, guys. Subscribe to my
channel for more tutorials, support me on
patreon so I can keep making content, and
as always feel free to email me: