# Photoelectric effect and energy of a photon

## Main Question or Discussion Point

Simple question: What happens to the energy of a photon that does not succeed in knocking off an electron??

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energy of unsuccessful photon

The photon is absorbed by an electron. The electron then gains energy but due to collisions with other atoms, it loses its energy and is unable to come out. However, some electrons still come out.

That is the reason why there is an expression for the max energy only. The case of no losses due to collisions.

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My book says that it is an all-or-none process; either an electron gains all the photons energy or none at all. Is this a bad description?

The description is correct.
An electron absorbs all of the energy contained in the photon. After that, the role of photon is over. We have an electron with a certain amount of energy. But the electron now might collide with other particles present in its vicinity and lose the kinetic energy it has gained as result of the absorption.

That's why it might come out with less energy or might not come out at all.
But the max. energy is fixed. It absorbs the photon and comes out with all the energy it gained with no loses.

ZapperZ
Staff Emeritus
broegger said:
Simple question: What happens to the energy of a photon that does not succeed in knocking off an electron??
When a photon enters a photocathode, a number of things can happen. The one that most people are familiar with is the photoemission of an electron. However, other process can also occur. The photon can be absorbed by the lattice itself, causing a conversion from photon to phonon/s, and the net effect being heat (but usually very, very miniscule). Even if the photon is absorbed to cause an electronic transition, the electron may, instead of escaping the material, be scattered in the "wrong" direction till it loses its energy to other electrons. This will also cause no photoemission.

Zz.

What does it mean by saying photons are absorbed by electrons.after absorbing the photons whereabouts in or on the electrons does the photon actually go?Also electrons are knocked off by photons,how does it prove that light is a particle?

ZapperZ
Staff Emeritus
saltrock said:
What does it mean by saying photons are absorbed by electrons.after absorbing the photons whereabouts in or on the electrons does the photon actually go?Also electrons are knocked off by photons,how does it prove that light is a particle?
I think I always have to make sure people correct their "phrases" or understanding of such a process. Please take note that when a photon is absorbed either by a solid or an atom, it is NOT the electron that is doing the "absorption". It is the WHOLE atom or the whole solid that is doing that. The net EFFECT may involve an electron making a higher energy transition, but it isn't the electron that is doing the absorption, because without the atom, or the bulk material, there won't be ANY energy levels to allow for such a transition.

In the case of the photoelectric effect, the WHOLE PHOTON is absorbed by the cathode. This energy is then converted into a transition by an electron to a higher energy state of the solid. If the energy state is higher than the solid's vacuum energy state, then there's a probability of that electron escaping from the bulk solid.

Now, the phenomena of electron being emitted from a solid due to light shining on it, by itself, doesn't "prove" that light is composed of photons. You need to understand the whole scenario of the photon picture, and a number of its predictions are tested via the photoelectric effect (i.e. you need to test for several criteria, not just one). All of these observations and tests indicate that they are CONSISTENT (not "proof") with the idea that light consists of photons.

Search the Hyperphysics site for the photoelectric effect and the kinds of tests involved in such an experiment, and why these observations are consisted with the photon picture.

There are other more stringent tests, including the multiphoton photoemission, that are even MORE compelling results supporting the photon picture.

Zz.

Right,that means photons are absorbed by the whole atom.Since photon is a particle,when it is absorbed by the atom where does it go.say,when glucose is broken down in the stomach,it is absorbed by the cells of our body.Absorbtion here means that glucose molecule gets into the cytoplasm through the cell membrane and remains there.Likewise when photons are absorbed by atom,where does that photon go?Does this mean the mass of the atom will be higher because photons have been absorbed.I am just confused by the term"absorbtion"What actually mean by saying this?sorry if i sounded bit crazy!!

ZapperZ
Staff Emeritus
saltrock said:
Right,that means photons are absorbed by the whole atom.Since photon is a particle,when it is absorbed by the atom where does it go.say,when glucose is broken down in the stomach,it is absorbed by the cells of our body.Absorbtion here means that glucose molecule gets into the cytoplasm through the cell membrane and remains there.Likewise when photons are absorbed by atom,where does that photon go?Does this mean the mass of the atom will be higher because photons have been absorbed.I am just confused by the term"absorbtion"What actually mean by saying this?sorry if i sounded bit crazy!!
It does sound a bit crazy, because you seem to think "photons" are conserved and must go somewhere. Photons are entities that carry energy (and spin and momentum). Energy can be converted to one form or another, and so can a photon. You would never ask what happened to this entity called "Potential Energy" after it is converted to something else. The same idea applies to a photon. It is NOT a particle like a ping-pong ball. It only exists as a DEFINITION of a finite amount of energy of an EM radiation. That's it!

Zz.

Thanks a lot Zapper.I am cool now.

Surely, couldn't an electron be hit by several photons in a given period of time and thus build up enough energy to escape even if the "light" is of low frequency? (eg - infra red)

Also, into what form of energy does the kinetic energy that is lost as a result of the work function become?

Thanks.

ZapperZ
Staff Emeritus
Cheman said:
Surely, couldn't an electron be hit by several photons in a given period of time and thus build up enough energy to escape even if the "light" is of low frequency? (eg - infra red)

Also, into what form of energy does the kinetic energy that is lost as a result of the work function become?

Thanks.
If you do a search on "multi-photon photoemission" on here, you will see that I have already mentioned this. Such an effect can only be appreciably observed when you have a light source of high intensity (i.e. high number of photons hitting per unit area per unit time). This is because things such as 2-photon photoemission is a 2nd order effect and thus, have a significantly lower cross-section than single-photon photoemission.

Also, we know when we have a multi-photon effect. The intensity vs. photocurrent relationship is not linear as in the single-photon production.

Zz.

$id Cheman said: Surely, couldn't an electron be hit by several photons in a given period of time and thus build up enough energy to escape even if the "light" is of low frequency? (eg - infra red) Also, into what form of energy does the kinetic energy that is lost as a result of the work function become? Thanks. Ill quote einsteins photoelectric equation the maximum kinetic energy of the escaped electron = h*f + Work function The work function is the minimum energy needed to remove the electron from the atom. So i presume its ionisation energy of that atom??? correct me if I am wrong. ZapperZ Staff Emeritus Science Advisor Education Advisor$id said:
Ill quote einsteins photoelectric equation

the maximum kinetic energy of the escaped electron = h*f + Work function

The work function is the minimum energy needed to remove the electron from the atom. So i presume its ionisation energy of that atom???

correct me if I am wrong.
You are not correct in that last part. Keep in mind that we are dealing with SOLIDS here, and not isolated atoms. In a solid, the valence shell of the atoms are no longer isolated - in fact, they form a valence BAND of that solid. That is how you get the conduction band in a metal. The conducton band isn't part of any individual atom, but rather part of the WHOLE solid.

Thus, it is not correct to assume that the work function reflects the ionization potential of the atoms that make up the solid.

Zz.