Question: Why does the number of electrons change as a result of photoelectric emission?
About the article "Photoelectric effect", section "Experimental observations of photoelectric emission".

Candidate passages:
Combining two scientific discoveries.
The invention of the photomultiplier is predicated upon two prior achievements, the discoveries of the photoelectric effect and secondary emission (i.e., the ability of electrons in a vacuum tube to cause the emission of additional electrons by striking an electrode). 
The photoelectric effect is the observation that many metals emit electrons when light shines upon them.
Electrons emitted in this manner may be called "photoelectrons".
According to classical electromagnetic theory, this effect can be attributed to the transfer of energy from the light to an electron in the metal.
From this perspective, an alteration in either the amplitude or wavelength of light would induce changes in the rate of emission of electrons from the metal.
Furthermore, according to this theory, a sufficiently dim light would be expected to show a lag time between the initial shining of its light and the subsequent emission of an electron.
However, the experimental results did not correlate with either of the two predictions made by this theory.
Instead, as it turns out, electrons are only dislodged by the photoelectric effect if light reaches or exceeds a threshold frequency, below which no electrons can be emitted from the metal regardless of the amplitude and temporal length of exposure of light.
To make sense of the fact that light can eject electrons even if its intensity is low, Albert Einstein proposed that a beam of light is not a wave propagating through space, but rather a collection of discrete wave packets (photons), each with energy "hf".
This shed light on Max Planck's previous discovery of the Planck relation () linking energy ("E") and frequency ("f") as arising from quantization of energy.
The factor "h" is known as the Planck constant.
In 1887, Heinrich Hertz discovered that electrodes illuminated with ultraviolet light create electric sparks more easily.
In 1905 Albert Einstein published a paper that explained experimental data from the photoelectric effect as being the result of light energy being carried in discrete quantized packets.
This discovery led to the quantum revolution.
Einstein was awarded the Nobel Prize in 1921 for "his discovery of the law of the photoelectric effect".
The photoelectric effect requires photons with energies from a few electronvolts to over 1 MeV in elements with a high atomic number.
Study of the photoelectric effect led to important steps in understanding the quantum nature of light and electrons and influenced the formation of the concept of wave–particle duality.
Other phenomena where light affects the movement of electric charges include the photoconductive effect (also known as photoconductivity or photoresistivity), the photovoltaic effect, and the photoelectrochemical effect. 
The theory of the photoelectric effect must explain the experimental observations of the emission of electrons from an illuminated metal surface.
For a given metal, there exists a certain minimum frequency of incident radiation below which no photoelectrons are emitted.
This frequency is called the "threshold frequency".
Increasing the frequency of the incident beam, keeping the number of incident photons fixed (this would result in a proportionate increase in energy) increases the maximum kinetic energy of the photoelectrons emitted.
Thus the stopping voltage increases.
The number of electrons also changes because the probability that each photon results in an emitted electron is a function of photon energy.
If the intensity of the incident radiation is increased, there is no effect on the kinetic energies of the photoelectrons.
Above the threshold frequency, the maximum kinetic energy of the emitted photoelectron depends on the frequency of the incident light, but is independent of the intensity of the incident light so long as the latter is not too high.
For a given metal and frequency of incident radiation, the rate at which photoelectrons are ejected is directly proportional to the intensity of the incident light.
Increase in intensity of incident beam (keeping the frequency fixed) increases the magnitude of the photoelectric current, though stopping voltage remains the same.
The time lag between the incidence of radiation and the emission of a photoelectron is very small, less than 10−9 second.
The direction of distribution of emitted electrons peaks in the direction of polarization (the direction of the electric field) of the incident light, if it is linearly polarized.
This internal conversion process is also not to be confused with the similar photoelectric effect, which also may occur with electron emissions associated with gamma radiation, in which an incident gamma photon emitted from a nucleus interacts with an electron, expelling the electron from the atom.
Thus, gamma photoelectric-effect electron emission may "also" cause high-speed electrons to be emitted from radioactive atoms without beta decay.
However, in internal conversion, the nucleus does not first emit an intermediate real gamma ray, and therefore need not change angular momentum or electric moment.
Also, electrons from the gamma photoelectric effect show a spread in energy, depending on how much energy has been imparted to the ejected electron by the gamma ray that interacts with it—an amount that is variable depending on the angle of gamma photon scattering from the electron.
Further, a gamma ray is still emitted in photoelectric processes, but one that possesses a fraction of the energy compared to the gamma ray that left the nucleus.
By contrast, in internal conversion, no gamma ray is emitted at all, and the electron energy is fixed at a single, typical value.
Auger electrons, which may also be produced after an internal conversion, arise from a mechanism that is different from that of internal conversion, but is analogous to it.
Internal conversion electrons arise when an intense electric dipole field inside the nucleus accelerates an electron that has penetrated the nucleus and removes it from the atom.
Auger electrons similarly arise when an electric field is produced within an atom's electron cloud due to loss of another electron, and this field again induces the acceleration and removal of yet another of the atom's atomic orbital electrons.
Like IC electrons, Auger electrons also emerge in a sharp energy peak.
The electron capture process also involves an inner shell electron, which in this case is retained in the nucleus (changing the atomic number) and leaving the atom (not the nucleus) in an excited state.
The atom can relax by X-ray emission and/or by Auger electron emission.
Unstable nuclei can usually decay through both internal conversion and electron capture processes. 
In a vacuum tube or electronic vacuum system, the cathode is a metal surface which emits free electrons into the evacuated space.
Since the electrons are attracted to the positive nuclei of the metal atoms, they normally stay inside the metal and require energy to leave it; this is called the "work function" of the metal.
Cathodes are induced to emit electrons by several mechanisms:  "Thermionic emission": The cathode can be heated.
The increased thermal motion of the metal atoms "knocks" electrons out of the surface, an effect called thermionic emission.
This technique is used in most vacuum tubes.
"Field electron emission": A strong electric field can be applied to the surface by placing an electrode with a high positive voltage near the cathode.
The positively-charged electrode attracts the electrons, causing some electrons to leave the cathode's surface.
This process is used in cold cathodes in some electron microscopes, and in microelectronics fabrication,  "Secondary emission": An electron, atom or molecule colliding with the surface of the cathode with enough energy can knock electrons out of the surface.
These electrons are called "secondary electrons".
This mechanism is used in gas-discharge lamps such as neon lamps.
"Photoelectric emission": Electrons can also be emitted from the electrodes of certain metals when light of frequency greater than the threshold frequency falls on it.
This effect is called photoelectric emission, and the electrons produced are called "photoelectrons".
This effect is used in phototubes and image intensifier tubes.
Cathodes can be divided into two types: