Talk: Photoelectric effect
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The point is whether we should define the photoelectric as "flow of electric current in a material when it is exposed to light" or a "emission of electrons from a material when it is exposed to light". I think the latter is preferable for the following reasons:
- The historical argument. I think that the first demonstration of the photoelectric effect was the observation of discharges (due to emission of electrons) between electrodes induced by ultra-violet light by Heinrich Hertz.
- A current will flow in a metal when exposed to ultra-violet light, but that is a secondary effect due to the holes left behind in the photoemission process. One can also knock electrons out of free atoms by photons, and in this case it does not make much sence to speak of current flowing.
- Photoemission spectroscopy, the detection of electrons from a solid (or liquid) with respect to kinetic energy and emission angle, is an important technique in solid state physics.
What do you think?
- I agree with the latter. A current is caused by energetic electrons, and is not always present in a demonstration of the effect. -Twinxor
As a reader of an encyclopedia, I would like to see reference to application of the scientific discoveries. Did this discovery brought us technologies that we used today or in the past? How did it change our life? Are solar power, digital cameras etc. based on this effect?
Please consider adding an "Applications" section to all entries that are related to science and technology.
My impression was that the current-flow effect was discovered first; if I am mistaken about that, and the first observations were simple discharges, then that might be a better starting place. I just wanted to make clear the distinction between Einstein and others' explanation of what causes the effect from the simple observation of the effect itself. I agree that mentioning applications would be good. Probably the earliest and best known was talking pictures: the soundtrack on a piece of movie film works by shining light through that portion film as it moves, stimulating an electrical current attached to sound amplifiers. --Lee Daniel Crocker
That's not the best example. The first optical soundtrack system, Movietone, used a photocell, which is a light-dependent resistor and has nothing to do with the photoelectric effect, and I believe that modern analogue optical soundtracks still use photocells. I think this is because photocells are more linear than photodiodes. Digital optical soundtracks, such as the Dolby digital system, are a different matter: they probably use photodiodes, because linearity is not necessary in a digital system. -- Heron 10:34, 10 Jul 2004 (UTC)
- Oops. I have just checked my facts, and found that my comment above was wrong. It seems that photocells and photodiodes both use the photoelectric effect. -- Heron 10:39, 10 Jul 2004 (UTC)
The emission of electrons is a more precise statement and should therefore replace "current flow." In fact, "current flow" brings to mind electrons (or charge) flowing within the material, but in the photoelectric effect the electrons are ejected and completely escape the material. --Carlos M.
"Emission of electrons" is clearly wrong, precisely because it is more accurate! Again, let me be clear: physicists (probably Hertz, but others might have noticed it earlier) observed some effect long before they knew that it was in fact the emission of electrons. It was later discovered what caused the effect, but the effect itself should be described as whatever the actual measurement or observation was that led us to figure it out; not what we currently understand as its cause--that's circular definition, and bad science. If what Hertz first measured was the discharge of a plate or a Leyden jar (as I now suspect), then define it that way. If what he first measured was a current flow (which I first assumed, perhaps erroneously), then say that. I'm not sure which it was, but I do know for sure that Hertz did not observe the emission of electrons. --LDC
Tesla / radiant energy
Tesla specifies that the exposed plate is insulated. It seems that the insulation should prevent light from hitting the metal plate directly. Or if transparent, the insulator should prevent electrons from being ejected. If so, how is this consistent with the photo-electric effect?
I have always been under the impression that static electricity actually drove Tesla's invention (triboelectrification between dust and the insulator). Alternatively, the device may be driven by photon induced ionization of the air. In either case, the capacitor may then be inductively charged.
Can anybody resolve this objection, or link to a careful discussion of Tesla's "radiant energy" device and the photoelectric effect?
- (William M. Connolley 19:30, 31 Aug 2004 (UTC)) I can't answer your point, but I have deleted the "Radiant energy" section, which appears to be dubious at best. If Tesla described/explained photoelectricity in 1901, as the section claims, he has great priority: Lenard's obs were not until 1902 [1]. So I don't believe this claim. At best, Tesla obtained a patent on a device using PE. He did not describe PE (important points being dep n freq not intensity) - or if he did, there is no evidence presented for this.
