# Photoelectric effect doubt

I've a doubt regarding photoelectric effect. It's said that photoelectric effect is proof for light to be a particle. But, when seen into the theory, relations between wavelength and kinetic energy, frequency and photoelectric current are explained. The means we have used wave characters like wavelength to explain photoelectric effect but still is accepted as proof for particle nature of light. Why? Anyone please explain.

mfb
Mentor
It's said that photoelectric effect is proof for light to be a particle.
No, it just proves light can show particle-like effects.
The means we have used wave characters like wavelength to explain photoelectric effect
The photoelectric effect is completely independent of wavelengths. You can purely work with the photon energies.

No, it just proves light can show particle-like effects.The photoelectric effect is completely independent of wavelengths. You can purely work with the photon energies.

But I've learnt recently that, if wavelength of incident radiation decreases, kinetic energy of photoelectron increases and if frequency increases, photoelectric current also increases. So as there is relation between wavelength and kinetic energy of photoelectron, frequency and photoelectric current, I got the doubt.

mfb
Mentor
But I've learnt recently that, if wavelength of incident radiation decreases, kinetic energy of photoelectron increases
While that is correct, you don't learn that from the photoelectric effect because it does not measure any wavelength.
and if frequency increases, photoelectric current also increases.
Not in general. It depends on the remaining setup, what happens with the intensity, your bias voltage, the geometry of everything and so on.

Drakkith
Staff Emeritus
The means we have used wave characters like wavelength to explain photoelectric effect but still is accepted as proof for particle nature of light. Why?

While the wavelength/frequency of the incoming light does affect the results of the experiment, the key is that there is a certain frequency/wavelength beyond which no current is detected, no matter the intensity of the incoming light.* This is odd because if light behaved like classical waves you should be able to simply increase the intensity of the light without changing the frequency and get ejected electrons, and the energy of each ejected electron should increase as the intensity of the light increases. But this doesn't happen. The simplest explanation that matches all observations is that light interacts with matter discretely, not continuously. By that I mean that energy from the light wave isn't absorbed in a continuous manner, as a classical wave would behave, but in discrete "packets", where each packet of energy is absorbed all at once. The amount of energy in each packet increase as frequency increases, explaining why the energy of the ejected electrons increases with the increasing frequency of the light. These packets of energy are called photons.

*Note that once the frequency of the light is high enough to eject electrons, the intensity of the incoming light can be reduced to nearly zero and you still get ejected electrons with the same energy as when the light intensity is much higher, albeit at a very low rate. This should not happen if light were just a classical wave and is further evidence that light has a particle-like behavior. If the light wave behaved classically, the electrons would absorb energy continuously over time, which would mean the electrons are behaving more like they do in a radio antenna. In an antenna, the electrons absorb energy in a near-continuous manner (owing to the very low energy per photon or radio waves) and this energy is dissipated as heat. No electrons are ejected from the antenna unless the intensity of the incoming radio waves is extremely high. So you can think of it as needing to deposit energy into the electrons faster than they can get rid of it as heat.

The photoelectric effect shows us that this behavior is only an approximation of a classical wave at low frequencies and that it fails when the frequency of the light is high enough. Classically, the incoming light would not eject any electrons below a certain intensity and increasing the intensity would always allow us to eject electrons. Instead, we get electrons regardless of intensity as long as the frequency is high enough and increasing the intensity of the light has no effect if the frequency is too low. As I said before, the best explanation is that light is not purely a classical wave and consists of photons.

Dr. Manoj
DrChinese
Gold Member
If you are interested in experimental proof regarding the particle nature of light:

http://people.whitman.edu/~beckmk/QM/grangier/Thorn_ajp.pdf

Observing the quantum behavior of light in an undergraduate laboratory
J. J. Thorn, M. S. Neel, V. W. Donato, G. S. Bergreen, R. E. Davies, and M. Beck
Abstract:
While the classical, wavelike behavior of light (interference and diffraction) has been easily observed in undergraduate laboratories for many years, explicit observation of the quantum nature of light (i.e., photons) is much more difficult. For example, while well-known phenomena such as the photoelectric effect and Compton scattering strongly suggest the existence of photons, they are not definitive proof of their existence. Here we present an experiment, suitable for an undergraduate laboratory, that unequivocally demonstrates the quantum nature of light. Spontaneously downconverted light is incident on a beamsplitter and the outputs are monitored with single-photon counting detectors. We observe a near absence of coincidence counts between the two detectors—a result inconsistent with a classical wave model of light, but consistent with a quantum description in which individual photons are incident on the beamsplitter. More explicitly, we measured the degree of second-order coherence between the outputs to be g^(2)(0) = 0.0177±0.0026, which violates the classical inequality g^(2)(0)>=1 by 377 standard deviations.

Dr. Manoj and Drakkith
George Jones
Staff Emeritus
Gold Member
If you are interested in experimental proof regarding the particle nature of light:

http://people.whitman.edu/~beckmk/QM/grangier/Thorn_ajp.pdf

Observing the quantum behavior of light in an undergraduate laboratory
J. J. Thorn, M. S. Neel, V. W. Donato, G. S. Bergreen, R. E. Davies, and M. Beck

Beck has also written a textbook on quantum mechanics, "Quantum Mechanics: Theory and Experiment"
https://www.amazon.com/dp/0199798125/?tag=pfamazon01-20

and in this book, Beck derives the results of the photoelectric effect by treating electromagnetism using classical waves and atoms using quantum mechanics.

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bhobba
While the wavelength/frequency of the incoming light does affect the results of the experiment, the key is that there is a certain frequency/wavelength beyond which no current is detected, no matter the intensity of the incoming light.* This is odd because if light behaved like classical waves you should be able to simply increase the intensity of the light without changing the frequency and get ejected electrons, and the energy of each ejected electron should increase as the intensity of the light increases. But this doesn't happen. The simplest explanation that matches all observations is that light interacts with matter discretely, not continuously. By that I mean that energy from the light wave isn't absorbed in a continuous manner, as a classical wave would behave, but in discrete "packets", where each packet of energy is absorbed all at once. The amount of energy in each packet increase as frequency increases, explaining why the energy of the ejected electrons increases with the increasing frequency of the light. These packets of energy are called photons.

*Note that once the frequency of the light is high enough to eject electrons, the intensity of the incoming light can be reduced to nearly zero and you still get ejected electrons with the same energy as when the light intensity is much higher, albeit at a very low rate. This should not happen if light were just a classical wave and is further evidence that light has a particle-like behavior. If the light wave behaved classically, the electrons would absorb energy continuously over time, which would mean the electrons are behaving more like they do in a radio antenna. In an antenna, the electrons absorb energy in a near-continuous manner (owing to the very low energy per photon or radio waves) and this energy is dissipated as heat. No electrons are ejected from the antenna unless the intensity of the incoming radio waves is extremely high. So you can think of it as needing to deposit energy into the electrons faster than they can get rid of it as heat.

The photoelectric effect shows us that this behavior is only an approximation of a classical wave at low frequencies and that it fails when the frequency of the light is high enough. Classically, the incoming light would not eject any electrons below a certain intensity and increasing the intensity would always allow us to eject electrons. Instead, we get electrons regardless of intensity as long as the frequency is high enough and increasing the intensity of the light has no effect if the frequency is too low. As I said before, the best explanation is that light is not purely a classical wave and consists of photons.

Thank you. I got whole idea.

A. Neumaier
This is odd because if light behaved like classical waves you should be able to simply increase the intensity of the light without changing the frequency and get ejected electrons, and the energy of each ejected electron should increase as the intensity of the light increases.
It is odd not because it contradicts the properties of classical light but because it contradicts the properties of classical electrons.

It is known since 1963 that quantum matter plus classical radiation already produces the photoeffect. See http://physics.stackexchange.com/a/131483/7924

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mfb and Drakkith
vanhees71