What Is the Photoelectric Effect?

The photoelectric effect is an important and magical phenomenon in physics. Under the irradiation of electromagnetic waves with a frequency higher than a certain frequency, the electrons in some substances will be excited by photons to form a current, that is, photo-generated electricity. The photoelectric phenomenon was discovered by the German physicist Hertz in 1887, and the correct explanation was proposed by Einstein. In the process of studying the photoelectric effect, physicists have a deeper understanding of the quantum properties of photons, which has a significant impact on the concept of wave-particle duality. ^[1]

Photoelectric effect

Photoelectric effect is

Photoelectric effect

The energy possessed by the photons in the beam is proportional to the frequency of the light. If the metal

In 1887,

Through a large number of experiments, it is concluded that the photoelectric effect has the following experimental rules:

1. Every metal has a limit when it comes to the photoelectric effect

Manufacture of photomultiplier tubes

When the calculation does not agree with the observation (that is, no electrons are emitted or the kinetic energy of the electrons is less than expected), it may be because the system is not completely efficient, and some of the energy becomes heat or radiation and is lost.

Light control appliances

The light control electrical appliances made of photocells can be used for automatic control, such as automatic counting, automatic alarm, automatic tracking, etc. The top right is the light control

The photoelectric effect phenomenon is

In the photoelectric effect, it is obviously necessary to release photoelectrons

The photoelectric effect is divided into:

In 1905, Einstein extended Planck's concept of quantization. He pointed out: Not only is the energy exchange between the black body and the radiation field quantized, but the radiation field itself is composed of discontinuous light quanta, and the energy of each light quantum and the frequency of the radiation field satisfy = h, that is, its energy is only related to The frequency of the photon is related to the intensity (amplitude).

Einstein's photoelectric effect equation

According to Einstein's theory of light quantum, the light that strikes a metal surface is essentially a stream of photons with energy = h. If the frequency of the irradiated light is too low, that is, the energy of each photon in the photon stream is small, when he irradiates the metal surface, the electron absorbs this photon, and the energy it increases by = h is still less than that of the electron leaving the metal surface The work function is required so that the electrons cannot be separated from the metal surface, so the photoelectric effect cannot be produced. If the frequency of the irradiated light is high enough that the energy absorbed by the electrons is sufficient to overcome the work function and escape from the metal surface, a photoelectric effect will occur. At this time, the relationship between the kinetic energy of the escaped electron, photon energy, and work function can be expressed as: photon energy-the energy required to remove an electron (work function) = the maximum initial kinetic energy of the emitted electron.

That is: _k ( max) = hv-W ₀

This is the Einstein photoelectric effect equation.

Where h is the Planck constant; v is the frequency of the incident photon

Photoelectric effect

Work function

is a work function, which refers to the minimum energy required to remove an electron from an atomic bond. The expression is as shown on the right, where f0 is the threshold frequency at which the photoelectric effect occurs, that is, the limit frequency; the work function is sometimes labeled with W or A. .

Kinetic energy expression

E (kmax) is the maximum kinetic energy of the emitted electrons, as shown on the right; m is the static mass of the emitted electrons; vm is the initial velocity when the emitted electrons escape.

Note: When this formula is inconsistent with the observation (that is, no electron is emitted or the kinetic energy of the electron is less than expected).

Experimental circuit

According to Einstein's photon quantum theory, the energy of the photoelectron in the photoelectric effect is determined by the frequency of the irradiated light, and has nothing to do with the intensity of the irradiated light, so it can explain the first two of the experimental rule. The limiting frequency means that the energy of the photon quantum just meets the photon quantum frequency to overcome the work function of the metal, and the energy required for the escape of different metal electrons is different, so the limiting frequency of different metals is different. For the third one, since the energy of the photon is sufficient, regardless of the light intensity (only determined by the number of photon), the electrons can escape immediately after absorbing the photon, so the photoelectric effect can be generated immediately without the accumulation process. When light hits the surface of a metal, the greater its intensity indicates that the more light quantum numbers it has, the more likely it is absorbed by the electrons in the metal, so it can explain why the number of emitted electrons is only related to the intensity of the light and to the light. Frequency-independent.