[Music]
here's a question that's plagued
physicists for centuries
what exactly is light when we talked
about this earlier we came to the
conclusion that light is a wave
which it is or at least light behaves
like a wave
when you use it in certain experiments
so for most of the 19th century it
seemed like the question had been
settled
physicists agreed light is a wave then
new discoveries made them start to
question that they started getting more
and more clues that light could also
behave
like a particle which led to the strange
concept that light was both a particle
and a wave this kickstarted the
development of a little something you
might have heard of
called quantum mechanics
[Music]
one of the most important clues that
light had to be more than just a wave
was what's known as the ultraviolet
catastrophe and that name really isn't
an exaggeration
the ultraviolet catastrophe was
disastrous for conventional thinking
about the physics of light we've talked
about how objects radiate heat
specifically the amount of heat they
radiate over time is proportional to
their temperature
raised to the fourth power but there's
more going on there than just heat
objects actually radiate energy that
covers a whole range of frequencies on
the electromagnetic spectrum
all different kinds of light now there's
this thing called a black body which is
basically the idealized version of a
radiating object
no true black bodies exist but in theory
they absorb
all incoming light without reflecting
any and radiate energy accordingly not
all the energy coming from a black body
has the same intensity you can predict
the intensity of the energy coming from
a black body or black body
radiation based on its temperature but
when physicists came up with an equation
for this intensity
using the idea that light is a wave they
ran into a big problem the equation they
came up with known as the rayleigh
gene's law
predicted that the higher the frequency
of the radiation and therefore the
shorter the wavelength
the higher the intensity and that
matched up with experimental results
really well
but only to a point once the frequency
of light got into the ultraviolet range
or higher
the rayleigh gene's law didn't fit the
results for experiments at
all instead experiments showed that
black bodies had a peak intensity based
on their temperature
at a certain frequency the light would
be at its most intense
and after that the intensity would
actually drop as the frequency increased
the warmer the object the higher the
frequency of the peak intensity
but there was always a peak it wasn't
supposed to be this way
even worse if you summed up the
contributions of higher and higher
frequencies to calculate the total power
emitted by a black body
the rayleigh gene's law predicted that
you'd find infinite power
which contradicts the principle of
conservation of energy this was the
ultraviolet catastrophe
something was clearly wrong about the
way physicists were thinking about light
as far as they knew intensity was only
supposed to keep getting stronger
as the frequency got higher so what were
they missing the catastrophe was
resolved
using the equation derived by german
physicist max planck
an equation that basically led to the
entire field of quantum mechanics the
equation known as planck's law was
actually very simple
but the concept it was based on was very
new planck's law says that
electromagnetic energy
takes the form of tiny discrete packets
called quanta
in other words at a certain point you
can't divide energy into anything
smaller than these packets and the
energy of each quantum is equal to the
frequency of the light
times a very small number called
planck's constant represented by the
letter h
if you take planck's law into account
when you try to predict the intensity of
blackbody radiation
you end up with an equation that
predicts the experimental results
perfectly
including those weird peak intensities
so the ultraviolet catastrophe was
resolved but now there was this whole
new idea that had physicists rethinking
everything
energy could only exist in discrete
packets quanta before
physicists thought energy was a kind of
continuous flow but it turned out that
at a certain point
you couldn't divide up energy into
smaller amounts and our old friend
einstein played a big part in reworking
physics using this new information and
he won a nobel prize for it in 1921
einstein argued that light energy
traveled in packets that we now call
photons which would essentially make
light behave like a particle
which was weird because remember there
had been lots of experiments that showed
that light behaved like a wave but
einstein suggested a way to prove
whether light traveled in these discrete
packets
an experiment involving the
photoelectric effect the photoelectric
effect describes what happens when you
shine a beam of light on a metal plate
electrons leave the plate and hit a
nearby collector creating a current
einstein realized that by studying the
way the electrons left the plate
physicists could learn a lot about the
properties of light because both the
wave theory and the particle theory of
light predict that light
knocks electrons out of the metal but
each theory has a different explanation
of why this happens and different
predictions when it comes to the details
of the experiment wave theory says that
when a light wave hits an electron
it exerts a force on the electron that
ejects it from the metal
according to this theory if you increase
the intensity of the light
you increase the strength of the
electric field hitting the electrons so
you eject more electrons and these
electrons have a higher speed and
achieve a higher maximum kinetic energy
which is the kinetic energy of the
fastest moving electrons leaving the
plate
one important thing to note here is that
according to wave theory the frequency
of the light shouldn't make a difference
only the intensity matters particle
theory on the other hand
says that electrons get ejected from the
metal when they're hit by
individual photons the photon transfers
its energy to the electron
which pops out of the metal and the
photon is destroyed in the process
but there's a minimum energy that the
photon needs to transfer
in order to get the electron to overcome
its attraction to the metal and pop out
that minimum energy is called the work
function w0
if the photon has less energy than the
work function then the electron won't go
anywhere but if the photon has more
energy than the work function
then some of the photons energy will be
used up to tear the electron away from
the metal
and the rest will give the electron
kinetic energy and some electrons will
be more
strongly attracted to the metal than
others but the electrons with the
maximum kinetic energy will be the ones
that took the bare minimum amount of
energy
to separate from the metal so according
to particle theory we can say that the
energy of the photon
is equal to the work function w0 plus
the maximum kinetic energy and the
energy of the photon is also equal to
planck's constant
times the frequency this equation tells
you that if you increase the frequency
of the light
the maximum kinetic energy of the
electrons should increase accordingly
and if you go below a certain frequency
f0
where planck's constant times f0 would
be equal to the work function
then you're not going to eject any
electrons at all this means that
increasing the intensity of the light
increases the number of electrons
ejected
but it doesn't affect their maximum
kinetic energy so if you want to know
whether the wave theory or the particle
theory is right
all you have to do is try a few simple
tests is there a cut-off frequency below
which electrons aren't ejected from the
metal
no matter how long you wait what happens
when you raise the frequency higher and
when you increase the intensity of the
light
does that affect the maximum kinetic
energy of the ejected electrons
turns out there is a cut-off frequency
and the higher the frequency is above
the cut-off
the higher the maximum kinetic energy is
of the electrons and sure enough
increasing the intensity of the light
only affects the number of electrons
ejected
it doesn't change their maximum kinetic
energy the results of all these tests
with a photoelectric effect
match up with the predictions of the
particle theory of light so photons
really exist light travels in discrete
packets
and behaves like a particle but what
about all those other experiments that
showed light behaving like a wave well
the thing is
light can behave like both in certain
circumstances it can behave like a
particle
in others it can behave like a wave this
is called the wave particle duality when
it comes to the physics of the very
small
your intuitive understanding of the
world just doesn't apply you can't
describe things like light using the
concepts you're used to that work on a
larger scale when you're trying to
explain something totally outside the
way you've directly
experienced the world you're going to
run into some brain bending physics and
the discovery of plank's law
along with the idea that light energy
travelled as discrete packets
turned into the foundation for the
concepts and equations that we use to
analyze the behavior
of the very small and the field of
physics which studies how quanta behave
is what we call quantum mechanics today
you learned about the ultraviolet
catastrophe and how it was resolved by
planck's law
we also talked about photons and how the
photoelectric effect
proves the particle nature of light
finally we discuss the wave particle
duality
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