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High school physics - NGSS
Course: High school physics - NGSS > Unit 6
Lesson 2: Electromagnetic radiation and matterThe photoelectric effect
Photoelectric materials emit electrons when they absorb light of a high-enough frequency. Created by Khan Academy.
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- When the photon gives off all its energy to the emitted electron that the photon dislodged in the photoelectric effect, what happens to the photon after this occurs?(3 votes)
- Energy, Energy!! The energy lost may just mean a exothermic or in this case less kinetic energy(2 votes)
- What does frequency even mean if we view light as photons rather than waves? Like, is it a measure of photons per second instead of cycles per second or are photons emitted in wave-like groups or what?(2 votes)
Video transcript
- [Instructor] The photoelectric effect is another one of these
cool things in physics that sounds like it should be sci-fi, but actually describes an
everyday phenomenon around us. This and related effects are
used for all sorts of things, like solar panels and cameras. And the name itself is the biggest hint into what's going on. Photo, photon, electric, electron. So what is the photoelectric effect? It's the emission of
electrons from a metal that has absorbed
electromagnetic radiation like light over a certain frequency. A particle of light is called a photon, and when a photon has enough energy, it can actually knock an electron free. What happens is that the photon collides with the metal surface,
hitting an electron. When these particles collide, some of the energy of the photon is used to dislodge the electron. That electron is then shot
out of the metal, or emitted. The rest of the photon's
energy is then transferred to the emitted electron. We know that energy
transfers between objects and is conserved in a system, so maybe this photoelectric
effect sounds simple, but that's actually why
it's such a big deal, so big that it's what Albert Einstein
received his Nobel Prize for. Why was this such a big deal? By understanding the photoelectric effect, we also learned a lot about the fundamental properties of light. Today, we know that light
can behave as both a particle and a wave, but that wasn't always known. When this effect was discovered, it was clear that light
had wave-like properties, but the photo electric effect
was a big piece of evidence that light could also
behave as a particle. It also showed that the
energy of those particles was related to the frequency of the light. So how did scientists figure all this out? Well, let's start by thinking about how to make a metal emit an electron. This sounds an awful lot like work, and we need energy to perform work. It turns out that electrons
have what is called a binding energy. Binding energy, which I'm
going to refer to as Eb, refers to the amount of energy needed to pull an electron away from
the atom that it's orbiting, or that it's bound to, and hence the name binding energy. Electrons are happy to
remain where they are, unless something gives them enough energy to overcome this binding energy and bounce them out of place. So the energy hitting an
electron has to be greater than its binding energy to cause emission. Scientists had observed
the photoelectric effect, but they wanted to know how
it worked, because, well, that's what scientists do. So over the years, various scientists designed
experiments that tested the effects of different types of light on different materials. And let's look at what they found. They found that if a light
was shining on a metal and the metal wasn't emitting electrons, changing the intensity of the
light didn't change things, but they did notice that
raising the frequency of the light did. So, for example, if you set up an experiment
where you're shining a lamp on a piece of metal and it's
not ejecting any electrons, shining two of those lamps on the metal won't change anything. But if you replace the bulb with one that emits a
higher frequency of light, this might change things. And this was a big hint
that light might be behaving as a particle. Why is that? If this was a normal wave, we would expect that by sending
more energy at the metal, by increasing the intensity, electrons would gain enough
energy to be emitted. But since that didn't happen, it was clear something else was going on. And that eventually led to the idea that light was composed of photons. And these photons need to individually transfer
enough energy to an electron that it can bounce out of place. So by changing the frequency of a light, we can change the energy of a photon. Once this energy is higher than that of the binding energy
we talked about earlier, it can knock an electron loose. Any extra energy then
contributes to the energy of the emitted electron. This is a big deal and enables us to do some pretty cool things. So let's go back to one of
the examples at the beginning, solar panels. They have these things called photo cells, which we're pretending
I'm drawing right now, and these photo cells use a version of the photoelectric effect
to generate electricity. When sunlight hits the
cells, they emit electrons. The cells are designed so that these electrons moving
around generate a voltage, and voltage can power devices. Now we can see why the
photoelectric effect is such a big deal. Not only did it revolutionize
our understanding of physics, but it and similar effects are
used in tons of technology.