How to describe the photoelectric effect... Proved
the quantum nature of light. Won Einstein a
Nobel Prize. Spawned a whole new field of physics.Revolutionize the way we view
the entire physical world!
Yeah it's a  pretty big deal.
Hey guys! Jade here. So the photoelectric
effect. The effect that proved that light was
made of particles. So you're thinking: what do
you mean like is made of particles?
I mean it doesn't look like a particle. But
more importantly in 1801 Thomas Young
proved that light was a wave! He had light  doing
all sorts of ways like behavior. Wave
interference, wave diffraction, there was
an interference pattern! All the evidence
pointed to light being a wave and not a
particle. It even proved supergenius
Isaac Newton wrong.
So now let's take a look at the
experiment that proved the quantum
nature of light, or that light is a
particle, either way. So we've got a
metal plate which we're going to call
the emitter. You'll see why in a sec. The
emitter plate is connected to the
negative terminal of the battery making the
emitter plate negatively charged. Now let's
place another metal plate opposite the
emitter which we're gonna call the
collector. The collector is connected to
the positive terminal of the battery
making it positively charged. Now the
electrons in the emitter want desperately
to jump across to the collector. Opposite
charges attract so the negatively
charged electrons want to get to the
positively charged to collector. But
there's a catch!
This setup is placed inside a vacuum.
Nothing's going anywhere, the electrons
stuck to the metal plates, no one can
move. Of course if the electrons were to
somehow break free of their metal prison
they would jump the gap no problem but
first they need to be given enough
energy. This is where our light source
comes in.
A light is shone on to the metal plate,
freeing the trapped electrons. The energy
carried in the light is transferred to
the electrons giving them the energy
they need to jump off the metal toward
the collector. We can measure how many
electrons are jumping off the emitter
and onto the collector by placing an
ammeter in the circuit. The flow of
electrons produces a current which is
recorded by the ammeter. The bigger the
current the more electrons jumping off. So
before we reveal thel experimental results
let's take a look at what classical
physics or Young's theory that light is a
wave predicts should happen. If light
were a wave... The amount of electrons
jumping off the emitter should depend on the
intensity of the light. Light of higher
intensity means a wave of  bigger amplitude
which means a wave carrying more energy. So a
higher intensity should produce a larger
current. So if we turn up the intensity
regardless of the frequency of the light
the electrons should be given enough
energy to break free.
Also if light were a wave this energy
transfer should be somewhat gradual so
there should be in time delay between
when the light hits the electron and
when the electron is ejected. Especially
at low frequencies. Another thing that
should happen is that as the wave
amplitude increases, more energy is being
carried in these waves, so the electrons
should have more kinetic energy as they
jump off the metal.
This means that as the intensity of the
light increases the electron should
travel with a higher speed toward
the collector.
Yeah none of that happened. Sorry Young.
It was observed that as soon as the
light hit the metal the electrons
jumped off the emitter instantaneously.
There was no gradual buildup of energy.
It was like the electrons we're getting
knocked off the surface. But the most
mind-boggling observation was that the
kinetic energy of the electrons was
found to depend on the frequency of the
light, not the intensity.
In fact for some frequencies no
electrons jumped off the metal at all.
The only way to explain this behavior is
that light is made of particles. It's
like if an electron were this melon and
different frequencies of light were this
delicious assortment of organic produce.
There's simply not enough energy in
this cherry tomato to knock the melon
off the table. You can turn up the
intensity as much as you like but no
matter how many cherry tomatoes you
throw that melon is not going to budge.
What about a Clementine? Still no. Now
let's try the lemon.
Yes! Looks like there was enough energy
in this lemon to knock the melon off the
edge.
This is what's called the threshold
frequency: the minimum frequency needed
to knock the electrons off the surface.
Frequencies above the threshold
frequency carry enough energy to knock
the electrons off the surface, but all
frequencies below the threshold
frequency,
well they just can't get the electrons
to budge. Kinetic energy of the electrons
were dependent on the frequency of the
light, not the intensity. The higher the
frequency the more kinetic energy.
It didn't matter the light was barely a
glow or so bright its blinding, the
kinetic energy changed only with the
frequency. From this is Einstein concluded that
the energy carried in one of these light
particles which he called photons was
also proportional to the frequency of
the light. More energy in a single photon
meant more energy being transferred to an
electron. The one thing that did depend
on the intensity was the electron
current. If the light shone was above the
threshold frequency a higher intensity
caused a larger current flow. And this
makes sense, I mean a larger intensity
means more photons, more photons means
more photon electron interactions, which
means more emitted electrons. Einsteins
discovery of photons won him the Nobel
Prize and kind of revolutionized physics.
"Oh okay so you're saying light was a
particle Einstein was right Young was
wrong. Light is a wave, stupid idea".
Well actually it turns out they were
both right. Light is a particle and a
wave.
Welcome to quantum physics! Light
sometimes acts like a particle and
sometimes acts like a wave. This phenomena
is widely accepted among physicists and
it's even given its own fancy name:
wave-particle duality. But it doesn't
just stop at light.  In the quantum world
particles and waves are doing this all
the time. An electron can have a
wavelength and
frequency and a wave can collapse into a
point like a particle. Don't get it? Don't
worry. As brilliant physicist Richard
Feynmann once said:  " I think I can safely say
that nobody understands quantum physics",
and he won the nobel prize in quantum
physics!
That's it guys thanks for watching I
hope you enjoyed this video. Over here is
my last video "The Proof That Light Is A
Wave". It's kind of like part 1 to this video
so make sure you watch that to get the
whole picture. Click the links to follow
me on Facebook Twitter and Instagram and
don't forget to subscribe so we can keep
learning about our world toegther. I need a better
catchphrase.
