PROFESSOR: So we’ve learned that
light has a particle nature. There’s a wave
property associated with light and electromagnetic
radiation, and also a particle nature– packets of energy being
carried along the wave. A brilliant experiment that
demonstrates that property is the photoelectric effect. And here’s how it works. You take a piece of metal. Now metal is an array of metal
atoms, and each of those atoms holds on rather loosely
to its outer electrons. That’s why the metal
conducts electricity. Those electrons are rather
free to move about the surface. Now, if you shine light on
that surface, what happens? Well, you can shine light
of certain wavelengths. Here we’ll bring in a rather
long wavelength– a red photon. And when a red
beam of light hits that, you’ll find, for many
metals, nothing happens. And even if you make the light
very bright, very intense, nothing happens. If you bring in a
green beam of light, this now has higher energy
photons, shorter wavelength, higher energy. What you find is that
electrons are indeed ejected. It’s as if photons are striking
electrons and kicking them off the metal. You bring in a more
intense green light, and you get more electrons. They don’t go away faster. You just get more electrons
with a brighter light, all with the same kinetic energy. You bring in blue light–
now higher energy photons– even a shorter
wavelength, and you find that electrons with
even higher kinetic energy are ejected. And again, the same
correlation with brightness– if you make the
light brighter, you get more electrons
per second released. So it’s as if the electrons
are caught in a well, and there’s an energy barrier
that holds the electrons next to the metal. You have to overcome that energy
barrier to eject the electron and release it from the metal. We’ll designate this threshold
energy, or this well depth, with the symbol Phi. So red light, the
photons of red light, don’t have enough energy to
even get out of the well. So it doesn’t matter if
there’s more of them. If there is more intense
light, more photons per second, still, none of
the electrons leave the well. Electrons of– or photons
in the green region– so shorter wavelength–
have enough energy to overcome this binding
energy of the electron being held to the metal and a
little bit of kinetic energy. More energy still
in blue photons ejects electrons with
even more kinetic energy. So it’s as if the
photons of light are coming in and
jostling electrons. I’m holding onto
this tennis ball. Photons are coming
in and jostling them. The higher energy the photon,
the more jostling occurs until you get to a photon that can
actually– higher energy now– break the photon
free of the metal. Can I have my tennis ball back? If you go to higher and higher
energies, bigger and bigger photons, in this
case, blue light, you’ll eject the electron
with more kinetic energy. Now brightness doesn’t matter. We said, well, brightness is
just more photons per second. That’s just peh, peh, peh, peh,
peh, peh, peh, peh, peh, peh, peh, peh, peh, peh– but not enough energy– one photon per electron to
eject any single electron. But big photons, high
energy, blue light, say, comes in and
slams that metal and really sends
the electron flying. [SOUND OF BALL HITTING OBJECTS] CREW: Ow! PROFESSOR: Ooh! [LAUGHS] Sorry, guys. [LAUGHS] High energy is what we have. So we can actually plot it. We can plot the kinetic
energy of the electron versus the frequency
of the light that we shine on the metal. And we know, up to
a certain frequency, no electrons are ejected. And then you’ll
reach that threshold. You’ll just give it– you’ll
just overcome the binding energy holding the
electron to the metal, and you’ll start to
eject the electrons. Then higher frequency light just
gives you more kinetic energy in the electron. The energy of the
photon, we know, is h Nu. So we can write
the kinetic energy that the electron has is the
energy that the photon comes in at, minus this binding energy. So all the excess
energy of the photon goes into kinetic energy. So you can write the both
energies in terms of photons. And you can realize
there is a minimum photon energy required to eject
the electron from the metal. If you look at different
metals, different metals have different
threshold frequencies. For instance, you
could have a metal that’s described by a blue
photon is the minimum photon that ejects an electron. And higher energy
photons give electrons with more kinetic energy. So this problem, the
photoelectric effect, helps us understand the
particle nature of light. And it’s actually Albert
Einstein, a genius, who looked at this problem. Scientists before the
photoelectric effect was understood and the particle
nature of light was understood, were taken– their metals, shining lights– and increasing the
intensity, figuring, I should knock off
more electrons. But increasing the intensity–
very bright light– didn’t do anything. And when you could
eject electron, increasing the intensity
didn’t increase the energy of the electrons. You just got more
electrons coming off with the same energy. Well, it takes a
genius, often, to look at a very troubling problem and
see it in a whole new light. And that’s what Einstein did. He said, well, that
looks like the light’s behaving like particles. It looks like little bits
of light are coming in. So a bright light is
just lots of bits. But they all have
the same energy. So those lots of bits eject
lots of electrons, each electron with the same energy. So the photoelectric
effect and Albert Einstein have helped us understand
the particle nature of light.

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Dennis Veasley

13 thoughts on “Photoelectric Effect-Tennis Ball”

  1. if i shined blue light onto a metal  continuously   would the metal   "run out" of electrons after a time

  2. What happened with the phi energy – threshold energy– ? Ekin = hv – phi, this means the energy of the electron is equal to the energy of the light minus the binding energy of the threshold. If there is a conservation energy, where that energy is? It happened the same with a rocket in the earth?After the rocked leave the earth, did it spend energy to get the threshold orbit? I mean the rocket energy of the rocket after leaving = possible total energy of the gas minus the energy to reach the stratosphere? I do not know. Can you please give me a clue?
    Other question: How you are sure that are more electrons with the same energy and no more energy per electron? How you know, or how can I see this? Is it a way to discover this? How you concluded this?
    Thanks in advance.

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