Understanding Photoelectric Effect with Einstein & Photons

[silverdiesel]

I am just trying to wrap my mind around this. Einstein explained the effect using photons, but why couldn't the electron get stripped by a wave. I understand that the electron will not get stripped until the energy is high enough, but I don't understand why a wave could not be resonsible. It seems to make perfect sense that a low energy wave will just wiggle the electron, but never have enough amplitute to actually remove the electron from the shell. Also, since light contains an oscilating electric field, doesn't it follow that it is that electric field that strips the electron? I just really don't understand what a photon is. I know it has the energy E=hf, but, take a radio wave for example, the energy is very low because f is very low, so the energy of the photon is very low... but would the photon also have the long length of the wave? The idea of photons seems to work with high energy light, but not the longer wavelengths.

 

[selfAdjoint]

The puzzle about the photoelectric effect, before Einstein's expanation, was that the electron stripping was not proportional to the light's intensity, but to its frequency. On any naive wave-stripping model, this was simply not accounted for. Einstein's invocation of Plank's formula, Energy = h times frequency, which Planck had applied to absorption and emission, to space traveling quanta of a radiation field explained this dependence.

Please do not confuse Einstein's quantum of radiation with a naive "particle" interpretation of radiation, either. Everything that Einstein (and Bohr and the rest of them) did was an anabasis from 19th century naivete.

 

[Gokul43201]

It seems to make perfect sense that a low energy wave will just wiggle the electron, but never have enough amplitute to actually remove the electron from the shell.

That was exactly the line of thought using wave theory. It was thought that increasing the amplitude of the wave (or the intensity of the beam) would impart more energy to the electron and hence, strip it. However, experiments showed that the threshold for stripping electrons had nothing to do with the amplitude of the wave. It was, however, clearly related to the frequency. And wave theory had no explanation for how you increased the energy by increasing the frequency.

 

[silverdiesel]

okay, I think I understand. Thanks for the help.

Does that mean, if you hit an atom with the right frequncy, no matter how small or large the intensity of the beam, you will see the same amount of electrons come off?

 

[selfAdjoint]

okay, I think I understand. Thanks for the help.

Does that mean, if you hit an atom with the right frequncy, no matter how small or large the intensity of the beam, you will see the same amount of electrons come off?

At least up to the level of measurement available in the early twentieth century. There is a lot of subtle modern work on photoemission that I am not an expert on; perhaps someone who knows more will post about it.

 

[Gokul43201]

Does that mean, if you hit an atom with the right frequncy, no matter how small or large the intensity of the beam, you will see the same amount of electrons come off?

Either SA has misunderstood your question, or I am.

If you hit a solid (consisting of several atoms) with a beam of high enough frequency, the rate at which electrons are ejected increases almost linearly with the intensity of the beam. The intensity is merely a measure of the number of photons passing through a given area in a certain time interval. The more high energy photons hitting the target, the more photoelectrons emitted.

 

[jtbell]

And if the light has a low enough frequency (or a long-enough wavelength), then a photon does not have enough energy to eject an electron from the metal. No matter matter how intense the light (no matter how many photons), no electrons come out. This cannot be made to fit with a pure wave model of light.