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Theories of the Universe

Photoelectric Effect Explained, the Quantum Strikes Again

Universal Constants

The photoelectric effect was first discovered by Heinrich Hertz in 1887. It works some-thing like this. When you connect a battery to two zinc plates enclosed in a vacuum, you can send an electric current through the empty space between the two plates without the use of a wire, simple by shining light on one of the plates. One of the plates is connected to the negative terminal of the battery and the other plate is connected to the positive terminal. If you shine a beam of light only on the negative plate, a meter hooked to the circuit will show that an electrical current is flowing.

Einstein was the first person to take seriously the idea of light quanta and to treat them as more than a mere mathematical trick used to explain the spectrum of black body radiation. Building on Planck's theory, Einstein was able in 1905 to explain another experimental puzzle, the photoelectric effect, by proposing that light is a stream of photon particles, each carrying one quantum of action. His theory that electrons are knocked out of a metal surface (such as zinc or copper plate) by photons, like coconuts being knocked off a fairground shelf by balls, was verified by the appearance of a photographic plate exposed to a very weak beam of light. The plate showed a patchwork of black spots where each photon had knocked out an electron, rather than a uniform gray exposure as would be expected if the light were a series of continuous waves.

Universal Constants

The frequency threshold refers to metal plates involved in the photoelectric effect. Every metal responds to a certain frequency of light at which it releases electrons. If the frequency of light is below the metal's threshold, it won't release electrons and current won't flow. But as soon as the frequency level is high enough, electrons will be released and current will flow in the photoelectric circuit.

The amplitude of a light wave, or any form of electromagnetic wave, is simply the height of the wave. The amplitude along with the wavelength and frequency will define the characteristics of any wave.

Let me present how the photoelectric effect was understood before it was discovered that light also came in chunks of quanta. Then we can see what Einstein discovered and how this revealed that light was a particle and not a wave.

Cosmonotes

Max Planck, Albert Einstein, and Neils Bohr made the first fundamental contributions to a new understanding of nature. Today the combined work of these three men, culminating in the Bohr model of the atom in 1913, is known as the old quantum theory. Three other individuals will later construct the new quantum theory, but that'll be found in another sidebar down the road.

  • When you shine ordinary white light on a plate, there is no current. In other words, no electrons are ejected no matter how bright the light.
  • If you increase the frequency of the light to a threshold frequency, usually in the ultraviolet, electrons are ejected and current flows.
  • When you go above the threshold, the higher the frequency of light, the electrons become more energetic.
  • Increasing the brightness of the light does not increase the energy of the electrons. However, it does increase the size of the current.

Are you a little confused? Perhaps rightfully so. But let's go on, I think it will be cleared up soon. For one thing, what we call brightness of a light source is an informal word for its intensity. As you already know, if you increase the intensity of a light source, you increase the energy output. According to Maxwell, the greater the energy in a light wave, the greater the amplitude. So you can think of increasing the intensity as increasing the amplitude of the wave.

But this makes the whole photoelectric effect very mysterious. No matter how much you increase the energy of white light (by increasing the amplitude of the waves), you can't knock any electrons out of the zinc plate. However, if you do increase the frequency, which classically doesn't change the energy of the light, you do knock electrons out. And if you continue to increase the light's frequency, you don't increase the number of electrons knocked out, but you do increase the energy of the electrons. On the other hand, increasing the brightness, which classically does increase the energy of the light, doesn't increase the energy of the electrons, but it does increase the number of ejected electrons. Is this any clearer? Maybe once you see what Einstein did, it'll explain the difference between the classical position and the new quantum.

Einstein explained that by taking the equation at the heart of Planck's description of black body radiation, E = hf, and applying it to electromagnetic radiation, the photoelectric effect could be understood if light itself came in definite chunks, or quanta, each with an energy, hf. It takes one light quantum to knock one electron out of the metal and, for a particular frequency, all the light quanta have the same energy, so all of the liberated electrons have the same energy. In a brighter light of the same color (frequency), there are more light quanta, but each quantum still carries the same energy; so more electrons, still with the same energy, are liberated. And when the frequency is increased, for example from yellow to violet, the frequency is higher, so each light quantum carries more energy, and the liberated electrons move faster, even if only a few electrons are released. Got it?

After that lengthy discussion of the photoelectric effect, I hope you got the gist of the outcome. It boils down to this. The classical understanding of light as a wave couldn't explain why the predictions of how light should behave didn't match the results. Einstein's explanation of light behaving as a particle of energy (light quanta or photon), instead of a wave, produced results that matched predictions. The main problem with this was that it led to the wave/particle duality of light that everyone had a tough time accepting. But in any case, quantum mechanics was starting to take off, and Neils Bohr is our next stop.

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Excerpted from The Complete Idiot's Guide to Theories of the Universe © 2001 by Gary F. Moring. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.

To order this book direct from the publisher, visit the Penguin USA website or call 1-800-253-6476. You can also purchase this book at Amazon.com and Barnes & Noble.


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