Pop quiz. How did Einstein win his Nobel prize? Was it for relativity? Nope, Einstein won the Nobel Prize in 1921 for the discovery of the photoelectric effect; how electrons are emitted from atoms when they absorb photons of light. But what is it? Let’s find out.
- Photoelectric Effect — University of Colorado
- Photoelectric Effect — University of Virginia
- Wave Particle Duality — GSU
- Battling Space Junk with a Tractor Beam of Charged Static Electricity — Wired
Transcription services provided by: GMR Transcription
Female Announcer: This episode of Astronomy Cast is brought to you by Swinburne Astronomy Online – the world’s longest running online astronomy degree program. Visit astronomy. swin. edu. au for more information.
Fraser Cain: Astronomy Cast episode 335 – The Photoelectric Effect. Welcome to astronomy cast; our weekly, facts-based journey through the cosmos. We’ll help you understand what we know; how we know what we know. My name is Frasier King. I’m the publisher of Universe Today. And with me is Doctor Pamela Gay, a professor at Southern Illinois University Edwardsville, and the director of Cosmoquest. Hey Pamela, how are you doing?
Pamela Gay: I’m doing well. We’re have windstorms here that seem to have deeply upset the dogs, so I apologize in advance for the upset interlude in the back.
Fraser Cain: I can hear some upset dogs. And now you’ve muted yourself. That’s perfect. They must be going really crazy.
You’re just nodding – no one can hear; she’s just nodding. They’re still going crazy? A lot of –
Pamela Gay: No, no; they stopped.
Fraser Cain: They’ll be back.
Pamela Gay: It’s winter, for those of you who are listening to this in the archive, and Frasier and I are having a Monday. My roof did bad things; it led to several inches of water. And Frasier’s garage –
Fraser Cain: Yep, my garage is flooded.
Pamela Gay: But, we have good things to announce for the future!
Fraser Cain: Yeah, the future will be bright. The future’s gonna be much better than it is right now.
Pamela Gay: And spring will come. So, on April 26/27 we’re going to repeat our hangout that’s on craziness, and do 36 straight hours of science content and fundraising to support Cosmoquest and all of our media programs, and all of our science programs, and all of our education programs –
We’re promoting this well in advance so you have no excuse for not saving the time to be with us for all 36 hours – or at least for a couple of them.
Fraser Cain: Yeah, no – we learned a lot of lessons – you guys learned a lot of lessons. What things really flew, and what things maybe didn’t fly, and what things you wanna do less of, and how to get people involved.
So I think it’s gonna be – hopefully people will see a whole new version of the hang-ma-thon. It’s gonna be fun. I’m in, and I’m sure we’re gonna see all of our space friends participating at various points during the show. So this is going to be great!
Pamela Gay: And I’m gonna start contacting people, so if you haven’t heard from me yet it’s because we’re recording this in advance, and if you’re listening to the live or in YouTube, I haven’t actually contacted you yet.
Fraser Cain: Okay; surprise! You’re being volunteered!
Alright, let’s get on with the show.
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Fraser Cain: Pop quiz – how did Einstein win his Nobel Prize? Was it for relativity? Nope. Einstein won the Nobel Prize in 1921 for the discover of the discover of the photoelectric effect – how electrons are emitted from atoms when they absorb photons of light. But what is it? Let’s find out. Nobel Prize, please!
So let’s go back to the story. If you ask most people – if you tell them, “Einstein one a Nobel Prize. What do you think it was for?” They’d be like, “E equals MC squared?” But no – photoelectric effect. So what’s going on?
Pamela Gay: So his very first Nobel Prize was this weird thing called the photoelectric effect that doesn’t really have anything to do with the whole E equals MC squared, gravity bending of light – none of that. What it has to do with, instead, is the weird realization that scientists have been having for several decades, at that point, that you could get electrons to be emitted from some sort of a metal surface if you shined the correct color of light on the surface.
And this was really confusing because at this point in time we didn’t really understand light at all. Or, they didn’t; I was very much not born yet. So scientist in general really didn’t have any understanding of what light was. There were people that thought that is was a wave, so you didn’t have discreet particles of light. Instead, you have this field that is waving through space, and if you increase the amount of light then that’s just a bigger wave. Right?
Then we had people that looked at it, instead, as a bunch of individual particles. But when you looked at it as individual particles people suddenly couldn’t explain a lot of things that had to do with diffraction, with interference effects, that seemed to only be related to waves.
So this was one of those, “Huh. How do we explain this,” moments, because if it’s a wave effect then you should either be able to send more red light at something – which is more energy – or, “What the heck’s going on? We’re just changing the color to blue,” ends up causing things to bounce off.
Fraser Cain: So particle or wave? They had to decide.
Pamela Gay: And the answer was one no one liked, and no one really likes today, and it’s both.
Fraser Cain: Right. And so what was – so they had this idea that light moved in waves in some instances. In other instances, light reacted like particles. So how does this play into the photoelectric effect, though?
Pamela Gay: It was actually when people were looking at the theory of black body radiation. They were trying to understand, “Why is it that?” When you turn up the temperature of something, if you make a measurement of how much light is given off at each color you see that it doesn’t keep getting brighter, and brighter, and brighter as you go to longer, and longer wavelengths. And as you go to shorter, and shorter wavelengths and higher, and higher frequencies – this was confusing because at the time all of the understanding that we had of light was that you should end up at this exponential growth, which was called the ultraviolet catastrophe.
And people got around – yeah, strange name, but that’s what it was called because, well, it was in the ultraviolet and you ended up with this exponential growth that no one could explain. Well, eventually it got explained, and that was a useful thing. The idea was you can’t really explain light unless you look at it as a bunch of individual particles where individual particles must have at least a certain amount of energy. And as energy goes up, it goes up discreetly.
Fraser Cain: In quanta, for example.
Pamela Gay: In quanta. As you look at this quanta the fact that it’s quantized – as we like to make up words – the fact that it’s quantized was what eventually allowed the black body curve to settle back down as you got to higher wavelengths.
Fraser Cain: So what’s going on then? Instead of it just – the amount of output increasing to infinity and causing the ultraviolet catastrophe –
Pamela Gay: Ultraviolet catastrophe –
Fraser Cain: – it’s got – what do we see now, as we’re observing the light and it’s going through this quantized state? What are we seeing?
Pamela Gay: What we see is what’s called a black body curve, because again, we’re not very original with how we name things. And when you have a low temperature object, you have a black body curve that never gets very bright that as you make a graph of number, or intensity, of the light coming off at different wavelengths, you see there’s a whole lot of photons coming off in the very red infrared wavelengths, it will eventually peek at whatever color of light corresponds to that temperature, and then it will drop back down. Cooler objects peek more to the red. As you heat something up more, and more, and more, as anyone who’s ever welded knows, the peek color – the color you see with your eye and the color that comes off of the object – shifts more and more to the blue.
Fraser Cain: So if you were just following our instincts on this – I take a black body – and don’t take responsibility for naming objects that were named before you were born Pamela; it’s really not your fault. If you take – we’re going to take some black ball – we’ll take a toaster. We’ll put it out in space and we’ll heat it up. And it’s gonna get hotter, and hotter, and hotter, and it’s gonna start to emit photons. So if you’re following your instincts, the toaster heats up, and it starts emitting photons of the color of light that matches the temperature that the toaster is.
We would be expecting to see photons streaming off this toaster that match the temperature that it set. But in fact, what we see, is we see this curve. We see some of the lower energy phots and some of the higher energy photons across this curve that is predicted by this black body radiation.
Pamela Gay: And what’s neat is the hotter and hotter the object gets, the more intense the light coming off of it is. You end up with a steeper peaked black body curve.
Fraser Cain: So if we push that temperature all the way up to thousands, millions, billions –of degrees, we’re still seeing lower energy photons –
Pamela Gay: The exact same shape of curve.
Fraser Cain: – and high energy photons – it’s still the same curve, it’s just shifted towards the shorter wavelengths and the higher energies.
Pamela Gay: And it was Plank who figured out, “This is what you need to do.” Gustav Kirchhoff – he’s the one who in 1860 came up with the phrase ‘black body’ to describe this phenomenon as you –
Fraser Cain: Just to be clear: not your fault.
Pamela Gay: It’s his fault; yes. And then it was Max Plant who eventually came up with a mathematical formula, involving a constant that later became Plank’s Constant, that describes how all of this works. Now, in coming up with this rule, he realized there’s a certain minimum energy – that there’s steps in the energy that you go through; everything’s quantized.
When Einstein looked at all of this experimental evidence that if you shine light on a surface just right, electrons bounce off. And he looked at Plank’s law – he started to think, “Well, what does it mean that we have quantized light?” You have the wavelength of the light, which defines the energy – so far so good – but if it’s a particle that wavelength is defining the energy of a specific particle.
If you have a wave the total energy of the wave is going to be the amplitude of the wave and – it all plays in together. But it’s not a wave that’s hitting the atoms on the surface – it’s a bunch of individual particles. So if I shine a bunch of low-energy – which means long wavelength, frequency is very slow – when that hits the surface each individual particle doesn’t have a lot of energy in it.
I can throw as many of these particles as I want at the surface, and the total energy hitting the surface is going to go up, and up, and up, because I’m hitting it with a bazillion particles. But if I’m an atom, because atoms are mostly empty the probability that I’m going to get hit by more than one these things – very, very low. So the atom’s just going, “Oh, I got hit by a very low energy photon. Oh! I just got hit by another very low energy photon. I don’t care.”
This is because the electrons in atoms are also quantized; they’re also restricted to specific energy levels. If you want to move an electron from one energy level to another you have to hit it with the precise energy required to make that jump.
Fraser Cain: If you’ve got one of these atoms in our toaster and it gets hit by a photon of light, if that photon of light doesn’t give enough energy to kick that atom up into a higher state, what happens to the photon? Does it get absorbed?
Pamela Gay: It just keeps going on with its day. There’s no interaction.
Fraser Cain: There’s not interaction so it could have gotten absorbed by the atom, but it didn’t bring enough to the table, didn’t commit.
Pamela Gay: And it was rejected.
Fraser Cain: It was rejected, which is back to that wave particle duality. It’s like a bullet – it’s like you’re shooting a bullet at a person and the bullet just gets to the person’s heart and, “Oh, you know what, it wasn’t a killing shot,” so it just passes through. But if it was a killing shot, then it would kill them. That’s just mind-bending.
Pamela Gay: Yes, and human beings – our brains tend to break a bit when trying to put all these pieces together. As we put all of these pieces together, what we start to realize is, “Okay, this starts to have consequences to what I need to do to do bad things to atoms.” In this case the bad thing we’re trying to do is get the electrons to go flying out of the atoms on the surface.
Fraser Cain: Well, it’s good if you want electrons, which we do.
Pamela Gay: Electrons are fine. But this can actually lead to a problem within other circumstances –but we’ll get to that in a bit.
Fraser Cain: Right, so we want to do the terrible things of getting an electron out of an atom. How do we need to do this?
Pamela Gay: What we need to do is slowly adjust the wavelength of the light we’re emitting until we match the energy of that wavelength to the energy needed to get an electron to go flying out of a surface.
Fraser Cain: And those are – I’m assuming just follow your periodic table of elements, because there’s math required and they all match up. Right? You can tune the right wavelengths. You can hit the right toaster, and you’re gonna get electrons streaming out of it.
Pamela Gay: Exactly. And what’s cool is we see this happening on the surface of the moon, where sunlight is capable of creating electrons flying off the surface, which lead to charged particles. We see this with a nice, friendly slab of metal if you throw light at it in the correct wavelength electrons come flying off of it. The wavelength needed completely correlates to whatever it is you’re trying to get the electrons off of.
Fraser Cain: What decides the energy level required to make that happen? Is it a more massive atom? Is it that it’s reflective? What causes that?
Pamela Gay: It’s the binding energy of the particular electron. So when you look at electrons, their binding energy is related to what orbital they’re in and how much energy it takes to get them to go from that orbital to being completely released from the atom. This is more, fancy quantum mechanics. It’s fairly easy to calculate for hydrogen; anything other than hydrogen it starts to become annoyingly difficult.
The nice thing is that, for instance, if you have a crystalline material you can often use x-rays to excite the crystalline material and get electrons to come flying out. With certain metals it just takes ultraviolet light. So we can produce free electrons and get a charged surface so when you remove the electron you’re left with a positive surface. That’s good for a number of things. For instance, if I want to detect light I can create a photon multiplier tube by making a sensor that at the color of light that I’m interested in it will cause electrons to be released. Then I just count those electrons and I use that as a surrogate for detecting the light.
Fraser Cain: Right –okay, okay! You need to look at the light – you’re using it as – it’s almost like an electron microscope. Right? Yeah.
So let’s talk about solar power, because I think that’s one of the ‘where the rubber hits the road’ with this effect. Right?
Pamela Gay: Exactly. So in the development of some solar panels – I can’t speak for all solar panels, so if there’s an exception to this don’t send us emails. What you end up with is you have a material that when it’s hit with sunlight, it is hopefully to a variety of different wavelengths of light – it’s sensitive will end up losing electrons, and this causes charge to flow. Now, over time the materials will degrade because you’re removing electrons, you’re created a charged surface, you do have to cycle the electricity all the way through so that they don’t stay charged forever otherwise you’ll run out of electrons and it just doesn’t work –
But this is a way to start the electricity flowing, charge your battery, complete the circuit.
Fraser Cain: Right, so essentially you are synchronizing the wavelength of sunlight with the material so that you’re in – you’re trying to find if you’re material’s [inaudible] [00:19:26] – you’re looking for the right kind of material that’s gonna be generating the most electrons that’s best synchronized with the wavelength of sunlight. And that if we lived in – for example, were constantly bathed in x-rays, our solar panels might look like crystals. Right?
Pamela Gay: So another neat way of using this is actually night vision goggles. What you have with night vision goggles are detectors that are sensitive to infrared radiation, and so something like a gallium chip, and when it gets hit with the infrared it, again, triggers charge and suddenly you’re seeing in infrared.
Fraser Cain: So, this is a way that, maybe, we could see – we talk about if you could see the sky with x-ray eyes, or gamma ray eyes, that that’s what we’re doing, for example we’re getting – if there was a way that you could have some kind of detector that’s releasing these electrons, and then mapping it, you could see what the world would look like. That’s really cool; that’s a really neat way to look at it.
So where – I think we talked a bit about Einstein. So where did Einstein pick up on this trail? Where did he figure it out, and carry the ball?
Pamela Gay: As near as we can tell it was a matter of there had been people working on this, literally, for decades, trying to figure out why it was that when you hit different surfaces with different colors of light you were able to get an emission – they weren’t fully clued in on electrons at this point. But initially they were able to figure out that there were negatively charged particles of some sort that came flying off of whatever was being illuminated.
People played with this phenomenon with a variety of different materials, a variety of different colors of light, and it was mostly experimentalists. Then you see Plank’s results and the beginning of quantanization. It seems that it was Einstein that basically combined Maxima’s equations for the electromagnetic effect with Plank’s concept of quantized light and black body radiation, and put together all of these pieces to realize what’s happening is you have a quanta of light with the precisely right energy to ionize one of these atoms coming along. He just put all of the pieces together and when he published this in 1905 it led to a, “Oh, that explains all of this,” kind of moment.
Fraser Cain: I love that this was during his miracle year, when he was working on all kinds of stuff. And this is – it feels like he was – not only was he so smart that he could work on relativity, but he was also like, “Oh, I need to win a Nobel Prize. What is something that I could just fix really quickly? Hold on – oh, this one. There. Solves all your problems. No one’s going to have a problem with it. Experiments are easy. Nobel Prize, please. Now let’s go back to relativity and –”
Pamela Gay: Well, and the experiments were already done –
Fraser Cain: Yeah, he was like, “Here is the answer. I’ve explained it. Nobel Prize.” No one would argue; done. So I just love that idea, that he just takes his level of genius to the next level.
Pamela Gay: It was actually Robert Millikan who, in 1914, said, “Okay, I’m going to very precisely verify everything that Einstein said,” and did the ultimate set of, “Dang it, light is a wave and particle set of experiments.
Fraser Cain: Was it some of the interference type experiments, or –? Yeah.
So now, obviously, we want to bring all of this back to astronomy, and we talked a bit about it. So how do astronomers use the photoelectric effect in their work?
Pamela Gay: It’s clearly part of all the detectors we use. We have to take it into account, unfortunately, when we’re building space craft because one of the problems that we deal with is the sun side of a spacecraft is experiencing the photoelectric effect – sunlight hits the spacecraft, electrons go flying off, you end up with a negatively charged – no, with a positively charged surface –
Fraser Cain: Right, this is the part we need to mention because if you get a positively charged surface, and you get momentum kicking off the space rock. Right?
Pamela Gay: Well, that’s less of an issue here so much as you end up with the shadowy parts due to the flow of electrons, and you end up with them being negatively charged. So shadow is negatively charted, then sunlight positively charged, and that flow of charge can do bad things to various sensitive instruments. There has to be a lot of care taken to figure out, “Okay, how do we protect things from these stray charges that will build up on the outside of our spacecraft?”
Fraser Cain: So they have to balance the electrical charge of the whole spacecraft?
Pamela Gay: Well, and use lots of insulation. That’s, at the end of the day –
Right, you can’t get rid of the photoelectric effect; sunlight hits, you’re going to end up with electrons flying off. But what you can do is realize that’s gonna happen and take extra care to isolate and insulate all of the spacecraft’s fragile circuitry.
Fraser Cain: Well, let’s talk about the momentum part, too, because that’s awesome. In addition to trying to deal with this charge, when mission planners are putting their trajectories for a spacecraft they have to account for the fact that it’s in sunlight and it’s gonna get pushed off of its trajectory because of the sunlight hitting it and the photoelectric effect.
Pamela Gay: So that’s not so much the photoelectric effect, in general, as you actually have light pressure. So this is the problem of lighter colored asteroids are going to experience more of a push than darker colored asteroids, so we can actually, in theory, go out and paint asteroids to move them around; which is just humorous.
With the photoelectric effect you have the light hitting the object and getting absorbed, so that’s a transfer of momentum. But, you have the momentum going this way, and you have the little tiny electron flying off this way, and the object continuing to move forward, but the light pressure still continuing to do that anyways.
Fraser Cain: Do you think there could be a way that spacecraft could harness this electricity? They harness it already with solar panels, but I wonder if there’s a way to sap up this extra charge differences that’s happening
Pamela Gay: Charge differences is one of those things where you can harness it for good in some instances, which is what solar rays are doing, but at the same time it tends to be an equalizing situation where you can only strip off so much charge from one set of electrons, you can only donate charge so much from another set – or, strip so many electrons off of atoms, rather. So it’s a self-limiting phenomenon.
Fraser Cain: There was an idea for cleaning up space junk that I had heard, that I think uses this – and tell me if I’m getting this wrong. But they would have a spacecraft fly out into places where there’s a lot of space junk. It would charge itself, negatively or positively, and then it would get close enough to these pieces of debris, which had gotten themselves charged up as well, get to certain distance, and then it would start to attract the positive side of the spacecraft would attract the negative side of the space junk. And the tug could then just move away with these – could change its position with these objects in tow, but it not actually have to touch them because they would be tumbling, or whatever.
You could, over time, have this tug move around, and actually gather up a lot of these objects – watch a bunch of them – they would have to track them down and gather up a bunch of this space debris without actually take it close enough to have to attach them or try to grab them or anything, and just let the difference in charge keep the attraction going.
Pamela Gay: The problem with something like that is when you say that something is tumbling, that action of tumbling means that you’re not going to end up with enough charge building up, because as it tumbles the part of it that’s – sorry, the dog is deeply offended again.
Fraser Cain: They don’t like tumbling spacecraft.
Pamela Gay: No, they don’t.
As the object is tumbling it’s constantly getting hit by sunlight, and so this doesn’t give it a chance to have sufficient charge building up in any one place. You’re much better off using magnetism.
Fraser Cain: I’m sure we’ll be doing more digging into the story, because it seemed a pretty fascinating idea to me. But I just thought I would just add that to the queue. I think we’re good. Well, thanks a lot, Pamela.
Pamela Gay: My pleasure.
Fraser Cain: Talk to you next week.
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Duration: 30 minutes