What do you know about the Photoelectric effect?

Answer 1

The photoelectric effect is the emission of electrons from matter upon the absorption of electromagnetic radiation, such as ultraviolet radiation or x-rays. An older term for the photoelectric effect was the Hertz effect, though this phrase has fallen out of current use
Upon exposing a metallic surface to electromagnetic radiation that is above the threshold frequency (which is specific to the type of surface and material), the photons are absorbed and current is produced. No electrons are emitted for radiation with a frequency below that of the threshold, as the electrons are unable to gain sufficient energy to overcome the electrostatic barrier presented by the termination of the crystalline surface (the material's work function). In 1905 it was known that the energy of the photoelectrons increased with increasing frequency of incident light, but the manner of the increase was not experimentally determined to be linear until 1915 when Robert Andrews Millikan showed that Einstein was correct [3].

By conservation of energy, the energy of the photon is absorbed by the electron and, if sufficient, the electron can escape from the material with a finite kinetic energy. A single photon can only eject a single electron, as the energy of one photon may only be absorbed by one electron. The electrons that are emitted are often termed photoelectrons.

The photoelectric effect helped further wave-particle duality, whereby physical systems (such as photons, in this case) display both wave-like and particle-like properties and behaviours, a concept that was used by the creators of quantum mechanics. The photoelectric effect was explained mathematically by Albert Einstein, who extended the work on quanta developed by Max Planck.


The photons of the light beam have a characteristic energy given by the wavelength of the light. In the photoemission process, if an electron absorbs the energy of one photon and has more energy than the work function, it is ejected from the material. If the photon energy is too low, however, the electron is unable to escape the surface of the material. Increasing the intensity of the light beam does not change the energy of the constituent photons, only their number, and thus the energy of the emitted electrons does not depend on the intensity of the incoming light.

Electrons can absorb energy from photons when irradiated, but they follow an "all or nothing" principle. All of the energy from one photon must be absorbed and used to liberate one electron from atomic binding, or the energy is re-emitted. If the photon is absorbed, some of the energy is used to liberate it from the atom, and the rest contributes to the electron's kinetic (moving) energy as a free particle.


In analysing the photoelectric effect quantitatively using Einstein's method, the following equivalent equations are used:

Energy of photon = Energy needed to remove an electron + Kinetic energy of the emitted electron


h is Planck's constant,
f is the frequency of the incident photon,
is the work function, or minimum energy required to remove an electron from atomic binding,
is the maximum kinetic energy of ejected electrons,
f0 is the threshold frequency for the photoelectric effect to occur,
m is the rest mass of the ejected electron, and
vm is the velocity of the ejected electron.
Note: If the photon's energy (hf) is not greater than the work function (φ), no electron will be emitted. The work function is sometimes denoted W.
In 1839, Alexandre Edmond Becquerel observed the photoelectric effect via an electrode in a conductive solution exposed to light. In 1873, Willoughby Smith found that selenium is photoconductive.


Heinrich Hertz, in 1887, made observations of the photoelectric effect and of the production and reception of electromagnetic (EM) waves, published in the journal Annalen der Physik. His receiver consisted of a coil with a spark gap, whereupon a spark would be seen upon detection of EM waves. He placed the apparatus in a darkened box in order to see the spark better; he observed, however, that the maximum spark length was reduced when in the box. A glass panel placed between the source of EM waves and the receiver absorbed ultraviolet radiation that assisted the electrons in jumping across the gap. When removed, the spark length would increase. He observed no decrease in spark length when he substituted quartz for glass, as quartz does not absorb UV radiation.

Hertz concluded his months of investigation and reported the results obtained. He did not further pursue investigation of this effect, nor did he make any attempt at explaining how the observed phenomenon was brought about.


In 1899, Joseph John Thomson investigated ultraviolet light in Crookes tubes. Influenced by the work of James Clerk Maxwell, Thomson deduced that cathode rays consisted of negatively charged particles, later called electrons, which he called "corpuscles". In the research, Thomson enclosed a metal plate (a cathode) in a vacuum tube, and exposed it to high frequency radiation. It was thought that the oscillating electromagnetic fields caused the atoms' field to resonate and, after reaching a certain amplitude, caused a subatomic "corpuscle" to be emitted, and current to be detected. The amount of this current varied with the intensity and color of the radiation. Larger radiation intensity or frequency would produce more current.

On November 5, 1901, Nikola Tesla received the U.S. Patent 685957 (Apparatus for the Utilization of Radiant Energy) that describes radiation charging and discharging conductors by "radiant energy". Tesla used this effect to charge a capacitor with energy by means of a conductive plate. The patent specified that the radiation included many different forms.


In 1902, Philipp von Lenard observed [2] the variation in electron energy with light frequency. He used a powerful electric arc lamp which enabled him to investigate large changes in intensity, and had sufficient power to enable him to investigate the variation of potential with light frequency. His experiment directly measured potentials, not electron kinetic energy: he found the electron energy by relating it to the maximum stopping potential (voltage) in a phototube. He found that the calculated maximum electron kinetic energy is determined by the frequency of the light. For example, an increase in frequency results in an increase in the maximum kinetic energy calculated for an electron upon liberation - ultraviolet radiation would require a higher applied stopping potential to stop current in a phototube than blue light. However Lenard's results were qualitative rather than quantitative because of the difficulty in performing the experiments: the experiments needed to be done on freshly cut metal so that the pure metal was observed, but it oxidised in a matter of minutes even in the partial vacuums he used. The current emitted by the surface was determined by the light's intensity, or brightness: doubling the intensity of the light doubled the number of electrons emitted from the surface. Lenard did not know of photons.

Albert Einstein's mathematical description in 1905 of how it was caused by absorption of what were later called photons, or quanta of light, in the interaction of light with the electrons in the substance, was contained in the paper named "On a Heuristic Viewpoint Concerning the Production and Transformation of Light". This paper proposed the simple description of "light quanta" (later called "photons") and showed how they could be used to explain such phenomena as the photoelectric effect. The simple explanation by Einstein in terms of absorption of single quanta of light explained the features of the phenomenon and helped explain the characteristic frequency. Einstein's explanation of the photoelectric effect won him the Nobel Prize (in Physics) of 1921.

The idea of light quanta was motivated by Max Planck's published law of black-body radiation ("On the Law of Distribution of Energy in the Normal Spectrum". Annalen der Physik 4 (1901)) by assuming that Hertzian oscillators could only exist at energies E proportional to the frequency f of the oscillator by E = hf, where h is Planck's constant. Einstein, by assuming that light actually consisted of discrete energy packets, wrote an equation for the photoelectric effect that fit experiments. This was an enormous theoretical leap and the reality of the light quanta was strongly resisted. The idea of light quanta contradicted the wave theory of light that followed naturally from James Clerk Maxwell's equations for electromagnetic behavior and, more generally, the assumption of infinite divisibility of energy in physical systems. Even after experiments showed that Einstein's equations for the photoelectric effect were accurate there was resistance to the idea of photons, since it appeared to contradict Maxwell's equations, which were believed to be well understood and well verified.

Einstein's work predicted that the energy of the ejected electrons would increase linearly with the frequency of the light. Perhaps surprisingly, that had not yet been tested. In 1905 it was known that the energy of the photoelectrons increased with increasing frequency of incident light, but the manner of the increase was not experimentally determined to be linear until 1915 when Robert Andrews Millikan showed that Einstein was correct .

The photoelectric effect helped propel the then-emerging concept of the dual nature of light, that light exhibits characteristics of waves and particles at different times. The effect was impossible to understand in terms of the classical wave description of light, as the energy of the emitted electrons did not depend on the intensity of the incident radiation. Classical theory predicted that the electrons could 'gather up' energy over a period of time, and then be emitted. For such a classical theory to work a pre-loaded state would need to persist in matter. The idea of the pre-loaded state was discussed in Millikan's book Electrons (+ & -) and in Compton and Allison's book X-Rays in Theory and Experiment. These ideas were abandoned.

Answer 2

It's been determined experimentally that when light shines on a metal surface, the surface emits electrons. For example, you can start a current in a circuit just by shining a light on a metal plate. Why do you think this happens?

Well...we were saying earlier that light is made up of electromagnetic waves, and that the waves carry energy. So if a wave of light hit an electron in one of the atoms in the metal, it might transfer enough energy to knock the electron out of its atom.


Okay. Now, if light is indeed composed of waves, as you suggest... What do you mean, "if light is composed of waves"? Is there another option?

Historically, light has sometimes been viewed as a particle rather than a wave; Newton, for example, thought of light this way. The particle view was pretty much discredited with Young's double slit experiment, which made things look as though light had to be a wave. But in the early 20th century, some physicists--Einstein, for one--began to examine the particle view of light again. Einstein noted that careful experiments involving the photoelectric effect could show whether light consists of particles or waves.

How? It seems to me that the photoelectric effect would still occur no matter which view is correct. Either way, the light would carry energy, so it would be able to knock electrons around.

Yes, you're right--but the details of the photoelectric effect come out differently depending on whether light consists of particles or waves. If it's waves, the energy contained in one of those waves should depend only on its amplitude--that is, on the intensity of the light. Other factors, like the frequency, should make no difference. So, for example, red light and ultraviolet light of the same intensity should knock out the same number of electrons, and the maximum kinetic energy of both sets of electrons should also be the same. Decrease the intensity, and you should get fewer electrons, flying out more slowly; if the light is too faint, you shouldn't get any electrons at all, no matter what frequency you're using.

That sounds reasonable enough to me. How would the effect change if you assume that light is made of particles?

I should give you some background information on this, first. It all began with some work on radiation by Max Planck...

Answer 3

The photoelectric effect is the emission of electrons from matter upon the absorption of electromagnetic radiation, such as ultraviolet radiation or x-rays. An older term for the photoelectric effect was the Hertz effect, though this phrase has fallen out of current use.

Answer 4

photoelectric effect is the emission of electrons from matter upon the absorption of electromagnetic radiation, such as ultraviolet radiation or x-rays. An older term for the photoelectric effect was the Hertz effect, though this phrase has fallen out of current use.

Answer 5

I only know how it applies to x-rays. The photoelectric effect is desirable and inversely proportional to compton scatter and kVp.

Answer 6

It's what Einstein got his Nobel prize for. It is all about how photons eject electrons from metals. Just wikipedia it.

Answer 7

Einstein won a Nobel Prize for it and it is evidence that light is a particle.

Answer 8

When light shines on a metal surface, the surface emits electrons. thats about all I know

Answer 9

Quite a lot thank you very much.

Answer 10

it;s eleactrical