The Photoelectric Effect

Transcription

1 The Photoelectric Effect 1 of 3 10/5/2007 8:36 AM The Photoelectric Effect What's the photoelectric effect? It's been determined experimentally that when light shines on a metal surface, the surface emits electrons. For example, you can start a current in a circuit just by shining a light on a metal plate. Why do you think this happens? Well...we were saying earlier that light is made up of electromagnetic waves, and that the waves carry energy. So if a wave of light hit an electron in one of the atoms in the metal, it might transfer enough energy to knock the electron out of its atom. Okay. Now, if light is indeed composed of waves, as you suggest... What do you mean, "if light is composed of waves"? Is there another option? Historically, light has sometimes been viewed as a particle rather than a wave; Newton, for example, thought of light this way. The particle view was

2 The Photoelectric Effect 2 of 3 10/5/2007 8:36 AM pretty much discredited with Young's double slit experiment, which made things look as though light had to be a wave. But in the early 20th century, some physicists--einstein, for one--began to examine the particle view of light again. Einstein noted that careful experiments involving the photoelectric effect could show whether light consists of particles or waves. How? It seems to me that the photoelectric effect would still occur no matter which view is correct. Either way, the light would carry energy, so it would be able to knock electrons around. Yes, you're right--but the details of the photoelectric effect come out differently depending on whether light consists of particles or waves. If it's waves, the energy contained in one of those waves should depend only on its amplitude--that is, on the intensity of the light. Other factors, like the frequency, should make no difference. So, for example, red light and ultraviolet light of the same intensity should knock out the same number of electrons, and the maximum kinetic energy of both sets of electrons should also be the same. Decrease the intensity, and you should get fewer electrons, flying out more slowly; if the light is too faint, you shouldn't get any electrons at all, no matter what frequency you're using. That sounds reasonable enough to me. How would the effect change if you assume that light is made of particles? I should give you some background information on this, first. It all began with some work on radiation by Max Planck...

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4 Planck's Constant and the Energy of a Photon 1 of 3 10/5/2007 8:36 AM Planck's Constant and the Energy of a Photon In 1900, Max Planck was working on the problem of how the radiation an object emits is related to its temperature. He came up with a formula that agreed very closely with experimental data, but the formula only made sense if he assumed that the energy of a vibrating molecule was quantized--that is, it could only take on certain values. The energy would have to be proportional to the frequency of vibration, and it seemed to come in little "chunks" of the frequency multiplied by a certain constant. This constant came to be known as Planck's constant, or h, and it has the value This doesn't make any sense to me. I think I'll go ask Dr. Mahan what J means. That's a pretty small constant. Yes, but it was an extremely radical idea to suggest that energy could only come in discrete lumps, even if the lumps were very small. Planck actually didn't realize how revolutionary his work was at the time; he thought he was just fudging the math to come up with the "right answer," and was convinced that someone else would come up with a better explanation for his formula. I guess Einstein must have taken him seriously,

5 Planck's Constant and the Energy of a Photon 2 of 3 10/5/2007 8:36 AM though. Quite seriously. Based on Planck's work, Einstein proposed that light also delivers its energy in chunks; light would then consist of little particles, or quanta, called photons, each with an energy of Planck's constant times its frequency. In that case, the frequency of the light would make a difference in the photoelectric effect. Exactly. Higher-frequency photons have more energy, so they should make the electrons come flying out faster; thus, switching to light with the same intensity but a higher frequency should increase the maximum kinetic energy of the emitted electrons. If you leave the frequency the same but crank up the intensity, more electrons should come out (because there are more photons to hit them), but they won't come out any faster, because each individual photon still has the same energy. And if the frequency is low enough, then none of the photons will have enough energy to knock an electron out of an atom. So if you use really low-frequency light, you shouldn't get any electrons, no matter how high the intensity is. Whereas if you use a high frequency, you should still knock out some electrons even if the intensity is very low. Quite right. Therefore, with a few simple measurements, the photoelectric effect would seem to be able to tell us whether light is in fact made up of particles or waves.

6 Planck's Constant and the Energy of a Photon 3 of 3 10/5/2007 8:36 AM So did someone do the experiment? Which way did it turn out? In , R.A. Millikan did a series of extremely careful experiments involving the photoelectric effect. He found that all of his results agreed exactly with Einstein's predictions about photons, not with the wave theory. Einstein actually won the Nobel Prize for his work on the photoelectric effect, not for his more famous theory of relativity. Then light is made of particles! But wait...what about the two-slit experiment? I don't see how light could make an interference pattern like that, unless it was made of waves. Yes, I'm afraid it's a bit more complicated than that. Some experimental results, like this one, seem to prove beyond all possible doubt that light consists of particles; others insist, just as irrefutably, that it's waves. We can only conclude that light is somehow both a wave and a particle--or that it's something else we can't quite visualize, which appears to us as one or the other depending on how we look at it. > th

7 Classic Two-Slit Experiment 1 of 2 10/5/2007 8:37 AM Classic Two-Slit Experiment OK, let's see what happens when we shine laser light through two slits and onto a wall. Press the light source button below to see the interference pattern... That's what we saw in class! But wait a second, this isn't exactly like the water waves we just looked at. Shouldn't the pattern be "rippling" like waves on a beach? Light waves ripple too fast for us to see. What we're seeing is the result of many light waves OK, so it makes sense that the pattern for light waves is just like the one for water. But where does the bizarre part

8 Classic Two-Slit Experiment 2 of 2 10/5/2007 8:37 AM piled up. come in? Well, to understand that we need to go a little bit further... > th

9 Electron Interference 1 of 3 10/5/2007 8:38 AM Electron Interference We've seen what happens when we shine light through two slits, or when water waves do something similar. But what do you think happens when solid objects go through the slits? I don't get it. What do you mean by solid objects? Like rocks? Solid things don't travel in waves, do they? Let's forget about waves for a second and just keep it simple. Dr. Feynman liked to talk about shooting a machine gun at an iron plate with two slots in it. If there were a concrete wall behind the iron plate, what kind of pattern do you think the bullets would make? Well, I would think bullets would just pile up behind the two slots. I guess they would bounce off the edges of the holes a little bit, so it wouldn't be real neat, but mostly they would just be in two areas. Right! The bullets Wait a second! But they might! Two bullets, one from each hole, might bounce into

10 Electron Interference 2 of 3 10/5/2007 8:38 AM don't interfere with each other like waves do... each other and knock each other all over the place. That's a kind of interference, right? Let's think about that. For two bullets to bump into each other would mean they left the gun at the same time. Do machine guns work like that? I hadn't thought about it, but I guess not. No matter how fast the machine gun seems to shoot, it's still just one bullet at a time. So there's no way the bullets could interfere. OK. Now we're going to try an experiment. Using our two slits from before, we're going to use an "electron gun" which shoots a steady stream of electrons, the same particles that orbit atoms, at a sensitive screen... Like a machine gun that shoots really small bullets. Yes. Each time an electron hits the screen it will make a green dot. Try switching it on...

11 Electron Interference 3 of 3 10/5/2007 8:38 AM Wait a second; it's slowly building up an interference pattern, just like with light. But that doesn't make sense. Are you sure the electrons aren't interfering with each other as they go through the slits? Maybe the electron gun doesn't work like a machine gun, and it shoots a bunch of electrons at once. OK, maybe so. How could we test that? Maybe we could turn down the electron gun until the electrons were coming out slowly enough for us to be sure it was one at a time. Lucky for us it does just that. Use the minus and plus keys on your keyboard to control the speed of the gun, and slow it down a lot. Then press your backspace key to clear the screen. Hey! The interference lines are building up anyway! How can it do that if the electrons are really like little bullets? What are the electrons interfering with? This is so strange... This is quantum physics. What does it mean? How do you explain it? We call it "particle/wave duality"... TO BE CONTINUED > st