I was watching a video explaining how the photoelectric effect shows the particle like nature of light. my understanding is that the frequency of the light determines the amount of energy delivered to the electron while the intensity (number of photons) determines the number of electrons emited. If thermionic emission can happen by heating a particle with a laser, it seems to me that higher intensity light is capable of emiting electrons even below the frecquency threshold. Does this mean that the nature of light can be reinterpreted as only wave like?

  • $\begingroup$ Some aspects of the photoelectric effect are actually not compatible at all with the particle model for light. For instance, the photoelectrons are primarily emitted in the direction of the field vector of the light wave, not in its direction of propagation. This is derived in detail in the book Quantum Theory of Radiation by W.Heitler , see archive.org/details/in.ernet.dli.2015.37198/page/n137/mode/2up $\endgroup$
    – Thomas
    Jul 9, 2022 at 11:22
  • $\begingroup$ this is why i asked on here! thank you this is exactly the sorta stuff i was looking for, cause the particle nature of light just makes no sense to me, it really feels like a work around for complicated wave dynamics $\endgroup$
    – Tess S
    Jul 10, 2022 at 15:00
  • $\begingroup$ It is actually known both theoretically and experimentally for almost 100 years that the photoelectrons are emitted primarily in the direction of the E-vector of the light wave (that is perpendicular to what one would expect from particle colliisons). See this experimental paper from 1931 drive.google.com/file/d/1ShAtM6_7HiKmEkiO5e9D3F2NvN3fCoUh/… $\endgroup$
    – Thomas
    Jul 10, 2022 at 19:48

1 Answer 1


Thermionic emission is the liberation of electrons from an electrode by virtue of its temperature (releasing of energy supplied by heat). This occurs because the thermal energy given to the charge carrier overcomes the work function of the material.

In solid-state physics, the work function (sometimes spelled workfunction) is the minimum thermodynamic work (i.e., energy) needed to remove an electron from a solid to a point in the vacuum immediately outside the solid surface.

You ask:

If thermionic emission can happen by heating a particle with light,

A particle, the electron, cannot be "heated" by light. It can interact with a photon that composes a light source. If you mean heating a material using lasers and watching the thermionic emission, there exist experiments studying this, (example) .

it seems to me that higher intensity light is capable of emiting electrons even below the frequency threshold

The photoelectric effect work functions are measured on cold metal surfaces. A heated solid , by definition of heat , will have a distribution of kinetic energy and for high temperatures of thermionic emission it is the high energy tail of the distributions that will provide enough kinetic energy to overcome the work function of the material studied ( note that the temperature of the material affects the work function ( example).

Light, classical electromagnetic waves, is composed out of a large number of photons, elementary particles in the mainstream standard model.

  • $\begingroup$ i editted my question to say laser, as you were right i meant laser. im wondering if thermionic emission shows that light does not have to be interpreted as being composed of photons as the light waves below the threshhold frequency add heat to the material which makes the work function lower, meaning lower frequencies can cause electron emission. basiclly can the photoelectric effect and thermionic emission be combined into 1 phenomenon? $\endgroup$
    – Tess S
    Jul 9, 2022 at 4:59
  • 1
    $\begingroup$ No, since the photoelectric effect is a surface effect of the lattice binding of electrons to the metal, whereas thermionic emission could happen even if the heat is provided in a different way than with radiation, as in lasers, as shown in the quote I give. The photoelectric does not depend on the maxwell boltzman distribution of molecular energy in the solid. $\endgroup$
    – anna v
    Jul 9, 2022 at 5:51

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