Talk:Photoelectric effect

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[edit] Old discussions (2004)

I have changed the first sentence:

The photoelectric effect is the emission of electrons from a usually metallic surface upon exposure to, and absorption of, electromagnetic radiation, such as visible light or ultraviolet radiation.

to:

The photoelectric effect is the emission of electrons from matter upon the absorption of electromagnetic radiation, such as visible light or ultraviolet radiation.

Usually is meaningless. Photoelectrons produced from non metalic solids, liquids and gasses is of as much interest as the photoelectric effect from metals. Saying that the matter is exposed to and then absorbs light is redundant. The absorption of a light implies that it has been exposed to the light.

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 I want to know why my question about Hertz as the discoverer of the photoelectric effect was not answered and erased instead.

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. --[[User::Lee Daniel Crocker|LDC]]

I see no a priori reason why the definition should be based on what led scientists to discover the effect.
  1. Such an approach is clearly anthrocentric, which some might not mind.
  2. Such a definition would mean that many phenomena would no longer be consider manifestations of the photoelectric effect. For example, there is no current caused in the levitation of moon dust.
  3. Such a convention would require rather severe changes in other terms. Do we want to define electric current as "the thing that happens in a frog's leg when placed on a moist tin dish and prodded with a lead knight"?

Phantym 19:27, 22 May 2005 (UTC)

[edit] 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.

Connolley, read the damn patents! Specifically ...

  • US685957 - Apparatus for the Utilization of Radiant Energy
  • US685958 - Method of Utilizing of Radiant Energy

The tube (marked as S) is cited in the patents as a Lenard tube or Roentgen tube. As to describing the PE, a basic description is in the patent ... if you read it!

(William M. Connolley 18:08, 20 Apr 2005 (UTC)) Tesla isn't in the std history of the photoelectirc effect. If you really think he belongs in the history, then he needs to be put in explicitly, not sneaked in via a "patents" section. And to do that, you need a proper source, because the patents don't seem explicit enough to me. Note that you have ignored my points above, viz: He did not describe PE (important points being dep n freq not intensity) - or if he did, there is no evidence presented for this.. I don't see anything in the patents addressing this - do you? I hope someone mopre competent will comment.
It's IRRELEVANT if Tesla isn't in the std history of the photoelectirc effect. He has been left out of a great many things in which he should be credited for. But the main source of information is from his patents ... so this probably would be better place for him. It's not "sneaked in", just a statement of facts.
The patents are pretty explicit, if you read them carefully (not just brush over them). Remember ... this (1901) is before much of the terminology is coin that we use today. The evidence is the 2 patents! NOTICE what the examiner wrote on the patent [it's in terms similar of which Thomson used].
As to describing the PE (I think you are menting depletion and frequency [dep n freq )... this is in both ... let's take US685957, in the 1st page of the description on lines 11-34 he mentions "vibrations" of "small wavelengths" ... he then goes on to note that HIS observations call for a new theory that is being advance in the patent about "minute particles" "electricified" [eg. electrons] and being thrown off.
If you can't comprehend this, you are not ignorant ... but an idiot
(William M. Connolley 19:39, 20 Apr 2005 (UTC)) It is highly relevant that T is not in the std history, since wiki is a summary of the general state of knowledge, not of your personal research. The patents do not seem at all explicit to me, and I note that you have not actually quoted the text from them: please do so if you think it is relevant. Your third point remains off the point. For your fourth point, you are reminded of the "no personal attacks" rule.
Wikipedia is a secondary source (smetimes a tertiary source) ... not just a summary of general knowledge ... eg., material that has generalization, analysis, synthesis, interpretation, or evaluation. This isn't my "primary" research ... but public knowledge from primary sources.
Patents are explicit and I have cited lines in the patent! (eg., 1st page of the description on lines 11-34)
I applogize for any "attack" ... but there are only so many conclusions to come to when facts are stated and then are ignored.

User:204.56.7.1 has put nonsense about Tesla discovering superconductivity on List of Tesla patents and Superconductivity. He seems impervious to facts. pstudier 20:45, 2005 Apr 20 (UTC)

He added stuff to solar cell too. - Omegatron 20:51, Apr 20, 2005 (UTC)

Ok, I didn't really have an opinion before (except skepticism of hero worship), but I've been reading a lot and this does look like the photoelectric effect to me. Yes, it's original research, but I'm finding some cool things to include in various articles while I do it.  :-)

"It is well known that certain radiations — such as those of ultra-violet light, cathodic, Roentgen rays [X-rays], or the like — possess the property of charging and discharging conductors of electricity, the discharge being particularly noticeable when the conductor upon which the rays impinge is negatively electrified." As long as the charging is positive and the discharging is of a negative object (as he seems to say in the second phrase), this is pretty much a statement of the photoelectric effect. However, his explanation for it ("that sources of such radiant energy throw off with great velocity minute particles of matter which are strongly electrified, and therefore capable of charging an electrical conductor") is wrong. He also says it is "well-known", though, so he isn't claiming to have discovered it; just demonstrating a way of capturing the energy.

A metal like aluminium has a work function of 4 eV. This means that to eject electrons from the surface through the photoelectric effect it needs to be hit by EM with a frequency above about 1 petahertz: math This is equivalent to a wavelength less than 310 nanometers math. This is in the range considered "near UV". So, through the photoelectric effect, UV and X-rays hitting a piece of metal will knock off electrons and give it a positive charge, as he said ("As the rays or supposed streams of matter generally convey a positive charge to the first condenser-terminal"). (But not visible light, so this has nothing to do with solar cells.)

As the charge gets bigger, though, the work function changes, so this only goes on until the charge is high enough that the electrons can't leave anymore. By connecting a large-value capacitor to the plate and the other side connected to a large charge reservoir (the earth), you can knock more electrons out of the plate than normal, because the capacitor "condenses" charge into a smaller space than the charge would normally fill by attracting it to opposite charges on the other plate. That's why they used to call them "condensors". So far, so good.

(Yes he does say something about the "receiver" being an "insulated plate or conducting-body", which made me think "coated in plastic" at first glance, but he also says "the surface should be clean and preferably highly polished or amalgamated", so I think he just means supported in a way that it is insulated from other objects, like you would say an insulated sphere of metal has a charge on it.)

  • Here is a sort of similar experiment: (fig. 3)

The one thing I'm not completely sure of yet is whether the photoelectric effect would create a net charge on a piece of metal in air.

  • This seems to indicate that yes, you can emit the electrons into the air (only if the metal is charged negatively in the first place?)
  • This says "Absorption in air by X-rays is minimal and the arriving X-rays have their full energy. Electrons, on the other hand, lose energy and are scattered by air". Since their testing depends on catching those electrons, they do their tests in a vacuum. In this case, though, the scattering of the electrons doesn't matter.
  • Tesla's (clever) metal foil switchy-thing discharger is specifically "inclosed in a receptacle, from which the air may be exhausted", which implies to me that the rest of the device is not. Hmmm...

But where do the electrons go? They just attach to the air and ionize it? Wouldn't they then drift back and stick to the metal and neutralize? Maybe it takes a while for them to drift back and neutralize, and you can discharge it into the earth before that happens (and then they will have to drift down to the earth to neutralize instead). Does a charged sphere eventually discharge by contact with the air? I'm used to thinking of circuits; not isolated charged objects. "The typical atmospheric molecule has an energy of about 0.03 eV." Aha! The electrons are tiny and also have >100 times as much energy. So maybe that is what happens. This paragraph should be considered much more speculative than the others, though.  :-) I'm really out of my element with the air charge stuff. - Omegatron 02:37, May 21, 2005 (UTC)

[edit] Counter Intuitive?

The dual nature of the photoelectric effect, wave or particle, made it very hard to accept. Does this count as Counter-intuitive?

Tabletop 10:56, 31 Mar 2005 (UTC)

I'm slightly confused. What dual nature of the photo-electric effect are you referring to? Do you mean the suggestion that (because of the photo-electric effect) light is a particle (which was previously thought of as a wave?) Or am I missing something completely? Pubuman 15:37, 10 August 2006 (UTC)

[edit] Moon dust

Are we sure that the moon dust gains its charge from the photoelectric effect and not from the charged particles streaming from the Sun? Enochlau 10:29, 21 Jun 2005 (UTC)

Did you check the references? Read this right after the balloon hair picture. - Omegatron 13:37, Jun 21, 2005 (UTC)
Hmm interesting, thanks.Enochlau 23:32, 21 Jun 2005 (UTC)

[edit] Removal of text

I removed the following piece of text.

[edit] Lee Hock Cheong's Photoelectric Effect using the Hexaton Atomic Model

As the proton is much larger than an electron, the probability to interact with a photon compared with an electron is therefore higher. When a nucleus absorbs a photon, the increase in energy will result in the displacement between the electron and the nucleus to be larger. This lets the orbital elecron escape from the magnetic and coulomb attraction of the nucleus with lesser kinetic energy than before. This is different from that presented by Albert Einstein, where he designated the electron as the vehicle for the absorption of photon as opposed to the proton forming the nucleus of the atom.

There are just 31 hits in total on Google, and this guy seems to be going against everything that is commonly accepted in the scientific circles. This sounds like one crackpot theory to me. (And we should check other pages to see if this guy's theory has creeped into them too.) Enochlau 01:41, 8 November 2005 (UTC)

Thanks! I may need to wait till this person's theory gets accepted by the scientific community before making further postings. Would anyone who has read the book by him or is actively engaged in atomic modeling theory give some comments. KJie.Neo 07:48, 8 November 2005 (UTC)

By someone else

There are no crack pot theories. Some theories are more correct in explaining some phenomenon until some other theory creeps up, which becomes more correct.

Hmmm, this theory seems rather far fetched to me. There are a few things to consider, firstly while it is true that electrons are far smaller than protons (about 1/2000th) it should be noted that the electrons cannot be attributed as point particles, or in fact as having a particular rigid shape. They are, for all intents and purposes of this explanation, a smear or cloud around the nucleus, thus the probability of interaction is not a simple matter of which particle is larger. Photons will have a hard time finding their way thru to the nucleus in the first place.

Secondly, If a nucleus was to be displaced what would cause the increase in the distance between the electron and the nucleus, The electrons would simply move with the nucleus, as per electrostatic forces, as the electrostatic force isn't reactive, i.e it is immediate. What would cause the electron to (essentially) stay in the same place (excuse the wording, an electron would never stay in the same place but for the sake of this argument...)

Thirdly, If the electron is emitted due to a change in displacement of the nucleus, then why would there be no dependence on the energy of the electron on the intensity of incident light? If the above theory were to hold, then there is no restoring force on the displaced nucleus, thus as more photons hit it will keep changing its displacement, and the greater change should then cause an electron with a higher binding energy to escape, which would mean the work function of the material would change with the intensity of the light as well.

Fourthly, if the electron simply escapes the bound of the electrostatic force (as this postulates) then why would the electron have a differing energy depending on the incident lights frequency. Surely all the electrons irrespective of the frequency, will have an energy coinciding with the bound state energy.

I could go on, but that will suffice. Pubuman 16:59, 10 August 2006 (UTC)

Since a single proton has a physical dimension of around 10E(-17)m, does it occur to you that an electron must have a physical dimension however small it may be? In fact, it is not that small considering that an electron has 1/1836 times the mass of a proton. Simple algebra would put the electron as having a physical dimension in the order of around 1/10 the physical dimension of the proton diameter if we consider the proton and electron as solid particles. If we consider it to be a hollow sphere, then the electron will be slightly smaller. In any case it will have a dimension not smaller than 10E(-19)m. The only problem is that our current instrument cannot measure the electron size accurately because the electron has another property that is governed by its spin. This property makes the measurement of the physical size of the electron extremely difficult. This is why no physicist is prepared to give the electron a physical dimension not because it has none.

Since an electron has a physical dimension, does it not occur to you that a proton that has a cross sectional area of roughly a 100 to a 1000 times that of an electron will have a greater probability of interacting with a photon that is fired at an atom? If that is the case, shouldn't the protons be a better candidate of interacting with a photon when it is fired at an atom?

If a proton inside the nucleus capture a photon, then this will essentially raise the rotational kinetic energy of the nucleus which commensurate with the amount of energy contain in that single photon. It will then cause the entire nucleus to rotate with a greater nuclear radius that varies with the photon energy. This will give rise to two effects simultaneously. At one phase of the orbital electron it will experience a stronger interactive electromagnetic force when it is in close proximity to the nucleus. At another it will experience a weaker interactive electromagnetic force when it is further away. The second phase of the electron rotation about the nucleus is the basis for the emission of a photo electron from the atom.

I am the author of the alternative theory to the explanation provided by Albert Einstein ie, the crack pot referred to by some of the contributors here. —Preceding unsigned comment added by 116.15.46.111 (talk) 03:11, 12 April 2008 (UTC)

[edit] This is photovoltaics, not the photoelectric effect

The discussion above, and the page we are discussing, is principally focused on photovoltaics, which is only one application of the photoelectric effect.

The photoelectric effect is actually the explanation for the phenomenon observed by Max Planck, wherein light of specific chroma ie wavelength, ie frequency, were emitted in certain conditions by certain substances.

This was found to be due to transitions in state of electrons held by atoms/molecules being examined.

Photovoltaics involves the entire free-ing of the electron, and the converse aborption of a free electron. But photoelectric effect covers those interactions, as well as the smaller transitions of state of an electron in orbit, where it remains in orbit at all times, and merely jumps from one level to another.

Please correct me if I'm wrong, or if I'm right, and I know I'm right, please help to repair the article.

It's not that the article has entirely bogus info, just that the focus is on a small part of the actual phenomenon, a part which has its own name as well. -- Yabbadabbadoo 13:38, 17 October 2006 (UTC)

I don't think that's correct - all my references say it's just the emission of electrons as described in the article. Can you provide a reference - website or book that supports what you said? enochlau (talk) 01:11, 18 November 2006 (UTC)

The issue of history also is of importance here. Alexandre Becquerel is mentioned as observing the photoelectric effect but is not credited with its discovery. It stands to reason that if an individual observes and describes the effect for the first time then he or she is the discoverer of the phenomena. Hertz's work helped develop the understanding of the effect but he should not be credited as the discoverer of the phenomena especially since Becquerel and his father are credited with the discovery in the wikipedia article on the photovoltaic effect. This needs to be clearly explained to avoid the confusion not only of the concepts themselves, but of the history of their discovery.

Got references? enochlau (talk) 00:54, 16 February 2007 (UTC)

[edit] Replace 'wavelength' with 'frequency'?

not to split photons but...

Under explanation: "The photons of the light beam have a characteristic energy given by the wavelength of the light"

Actually, isn't it the frequency that defines the energy? The wavelength varies depending on the medium but the medium does not affect the energy of the photon.

~Peter —The preceding unsigned comment was added by 129.237.121.29 (talk) 19:05, 4 January 2007 (UTC).

Correct; the frequency is what determines the energy. I've changed it. Uberdude85 20:41, 11 March 2007 (UTC)

[edit] Replace image

Hello everyone I would like to know if the lead image of the article

be substituted by an animated pic

LegalEagle 05:58, 25 February 2007 (UTC)

  • Strong oppose; animated images are generally obnoxious and seldom add much useful, as this one so aptly demonstrates. Dicklyon 06:00, 25 February 2007 (UTC)
  • Oppose; for the same reasons as above. Uberdude85 21:04, 11 March 2007 (UTC)

[edit] Photoelectrons or electrons?

In the first sentence defining the photoelectric effect it says "photoelectrons are emitted". I propose that this be changed to "electrons are emitted" as the "photo" prefix is not necessary and may leave the reader wondering "what's so special about these photoelectrons?". As this is the first line of the article it is important to be succinct, but also I thought I would canvas opinion before changing it. Uberdude85 20:51, 11 March 2007 (UTC)

[edit] Einstein was correct

The end of the second paragraph states that Einstein was correct, but it does not say what he was correct about.

[edit] Photons or electrons ejected?

In the first paragraph in the 'Explanation' section, there's this sentence:

Increasing the intensity of the light beam does not change the energy of the constituent photons, only the number of photons ejected.

I'm not physicist, so it's likely that I'm mistaken; but it appears to me that it's electrons that are ejected, not photons. If I am correct, then (in addition to changing 'photons ejected' to 'electrons ejected') maybe some clarification is in order, regarding which or how many electrons are ejected (e.g., presumably not all from the same atom). I'm not changing anything myself because I'm too clueless about physics.

Amichai Schreiber 13:01, 16 July 2007 (UTC)

Thanks for pointing that out. enochlau (talk) 13:39, 16 July 2007 (UTC)

[edit] Photocopier

From what I understand a photocopier makes use of a photo sensitive drum as an important part of its copying cycle. Am I correct in assuming that this drum exploits the photoelectric effect? If so i believe that the photocopier should be added to the list of applications as this effect is central to the photocopier's operation. Aamackie (talk) 02:31, 10 January 2008 (UTC) Sorry, just did a little research, it doesn't use the photoelectric effect. Aamackie (talk) 01:52, 17 January 2008 (UTC)