Photoelectric Effect

[JoeLight]

Why do we assume a photon is a particle that is dislodging electrons from the metal surface. Couldn't the vibration of the electromagnetic field be responsible for the freeing of electrons and at low frequencies there just simply enough of a vibration to free the electron regardless of the intensity? I guess I just don't understand why the photoelectric effect means light has to behave as particle.

[Strange]

It doesn't really mean it behaves as a "particle" (depending what you think the word means). More importantly it tells us that the energy of the electromagnetic radiation is quantised.

 

You need a certain amount of energy to remove an electron from the material. If you view light classically, then you should be able to get enough energy by either increasing the amplitude or increasing the frequency. If the light has too low a frequency, it doesn't matter how bright it is. This shows that it is not just the total incident energy that matters, but the energy in one photon.

 

I have just checked, and the Wikipedia page explains it better than I can!

http://en.wikipedia.org/wiki/Photoelectric_effect

 

Note the reference to Planck. He had already come up with the idea that light must be quantised to solve the "ultraviolet catastrophe".

http://en.wikipedia.org/wiki/Ultraviolet_catastrophe

[JoeLight]

It doesn't really mean it behaves as a "particle" (depending what you think the word means). More importantly it tells us that the energy of the electromagnetic radiation is quantised.

 

You need a certain amount of energy to remove an electron from the material. If you view light classically, then you should be able to get enough energy by either increasing the amplitude or increasing the frequency. If the light has too low a frequency, it doesn't matter how bright it is. This shows that it is not just the total incident energy that matters, but the energy in one photon.

 

We view increasing the amplitude as increasing the displacement of the crest from the resting state. How do we know were not simply increasing the number of waves rather than changing the characteristics of them. so instead of doubling the amplitude of 1 wave your actually adding a second which still doubles the energy output without with changing the peak of the crest? Is it possible to or has anyone ever performed experiments with the photoelectric effect using single photons or waves of light?

Edited August 5, 2014 by JoeLight

[Sensei]

Why do we assume a photon is a particle

 

Because it's particle.

 

Are you aware that we can create electron and positron pair from single photon that has energy > 1.022 MeV?

 

y -> e- + e+

 

Typically photon has to hit some nucleus to produce pair.

 

The higher energy of photon can produce other pairs of particles.

 

Photons with extreme energies are creating shower of new particles.

 

http://en.wikipedia.org/wiki/Ultra-high-energy_gamma_ray

 

 

I guess I just don't understand why the photoelectric effect means light has to behave as particle.

Photoelectric effect is just one of many effects confirming particle nature of light.

Photon-atom interactions video:

 

[Janus]

We view increasing the amplitude as increasing the displacement of the crest from the resting state. How do we know were not simply increasing the number of waves rather than changing the characteristics of them. so instead of doubling the amplitude of 1 wave your actually adding a second which still doubles the energy output without with changing the peak of the crest? Is it possible to or has anyone ever performed experiments with the photoelectric effect using single photons or waves of light?

An equivalent is done by turning down the intensity of the light at a frequency the kicks electrons free. If the light was not arriving as quanta, you would get to a point where the energy would just not be enough to kick any electrons free. However, what we see is that while the rate drops, we still have electrons knocked loose.

 

An Analogy would be like this:

 

You have a ping pong ball sitting in a small depression. You fire a stream of water at it to knock it out. At first, we expect the stream of water to knock out the ping pong ball. Then we start to decrease the flow of the stream. At some point, we expect the force of the stream to be less than that needed to knock free the ball. But even after we decrease below this, the ball still gets knocked loose.

 

We know how much water is coming out per sec, and we know how much force that water will produce as a continuous stream and it works out to be less than that needed to knock loose the ball.

 

But, if the water was not a continuous stream, but instead a series of "spurts", We have the same average flow rate over a given time period, but it is divided up into high flow spurts spaced apart from each other. It becomes the difference between a steady push and a series of kicks. The total energy might be the same,but whereas a steady push for 1 sec can't dislodge the ball, the same energy delivered as a single kick can.

 

So the same is with the Photoelectric effect, we can turn down the intensity to the point where a steady flow of waves would not be enough to knock free an electron, but the same energy delivered as discrete bundles could.

[Strange]

We view increasing the amplitude as increasing the displacement of the crest from the resting state. How do we know were not simply increasing the number of waves rather than changing the characteristics of them.

 

Because that is how "amplitude" is defined. You apparently want to redefine it to mean "frequency". Fine, but then we need to introduce another term to indicate the height/strength/loudness of the wave.

[studiot]

How do we know were not simply increasing the number of waves rather than changing the characteristics of them.

 

Because shining two light sources on the target produces a single combined light beam not two separate ones side by side.

 

And of course a single light source produces only one light beam.

[JoeLight]

 

Because shining two light sources on the target produces a single combined light beam not two separate ones side by side.

 

And of course a single light source produces only one light beam.

Thank you for answering my question.

What evidence, or experiments, have confirmed that the height of wave is not a constant. Have we been able to see it as a variable or are we assuming it can change? If so could you point me in that direction so I can read about it? If anyone has good sources of information on this stuff please share. When you start searching the wonderfully wide web lol you get a lot of useless information. I'm really trying to learn here.

[studiot]

Since light is just a form of EM wave, in wave terms, one of the simplest to measure might be to use radio waves rather than light and use lecher lines.

 

Wiki offers them to measure wavelength but they can also be used to measure amplitude.

 

http://en.wikipedia.org/wiki/Lecher_lines

 

 

 

Of course a light intensity meter provides a measure of amplitude.

Edited August 5, 2014 by studiot

[imatfaal]

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