Quantization of Energy Part 2: Photons, Electrons, and Wave-Particle Duality

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.

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