The photoelectric effect is a very important and magical phenomenon. To put it simply, it means that under the irradiation of photons of a certain frequency, the electrons inside certain substances will be excited by the photons to form an electric current, which is converted from energy to From a perspective, this is a process of photoelectricity, the conversion of light energy into electrical energy.
The formula of photoelectric effect: hv=ek+w.
Among them, hv is the energy of the photon with light frequency v, h is Planck’s constant, v is the frequency of the photon, ek is the maximum initial kinetic energy of the electron, and w is the energy of the excited substance. Escape from work.
1. Basic properties of the photoelectric effect
1. Every metal has a limiting frequency, or cut-off frequency, when it produces the photoelectric effect, that is, the frequency of the irradiated light cannot be lower than a certain critical value. The corresponding wavelength is called the limiting wavelength, or red limit wavelength. When the frequency of the incident light is lower than the limiting frequency, no matter how strong the light is, electrons cannot escape.
2. The speed of photoelectrons generated in the photoelectric effect is related to the frequency of light and has nothing to do with light intensity.
3. The instantaneous nature of the photoelectric effect. The experiment found that a photocurrent is generated almost immediately when the metal is illuminated, and the response time does not exceed ten negative ninth seconds (?1ns?).
4. The intensity of the incident light only affects the intensity of the photocurrent, that is, it only affects the number of photoelectrons escaping per unit area per unit time.
2. The work function of the photoelectric effect
The work function refers to the work that must be done by electrons to overcome the constraints when irradiating a metal with light to escape from the metal surface.
The commonly used unit is electron volt eV. The work function of metal materials is not only related to the properties of the material, but also related to the state of the metal surface. Coating different materials on the metal surface can change the work function of the metal. size. When the external light energy is lower than the work function, the photoelectric effect will not occur.
3. Several points that need to be paid attention to in understanding the photoelectric effect
1. It reflects the particle nature.
2. The condition for the photoelectric effect to occur is that the photon frequency must be greater than or equal to the cut-off frequency, that is, the photon energy must be large enough.
3. The photoelectric effect occurs in a very short time without lag.
4. One photon corresponds to one electron, and the excited one is called a photoelectron.
5. The increase in light intensity refers to the increase in the number of photons per unit time. An increase in light intensity will increase the size of the current, but will not increase the initial kinetic energy of the electrons.
Extended information:
The phenomenon of the photoelectric effect was accidentally discovered by Hertz while doing spark discharge experiments that confirmed Maxwell's electromagnetic theory, and this phenomenon became a breakthrough in Maxwell's electromagnetic theory. an important piece of evidence.
The light quantum explanation given by Einstein when studying the photoelectric effect not only promoted Planck's quantum theory, but also proved that wave-particle duality is not just for energy, but that light radiation itself is also quantized. , and at the same time provides natural scientific evidence for the law of the unity of opposites in materialist dialectics, which has immeasurable philosophical significance.
This theory also laid the foundation for Bohr's atomic theory and de Broglie's matter wave theory. Millikan's quantitative experimental research not only proved the light quantum theory from an experimental point of view, but also provided evidence for wave theory. Er's atomic theory provides evidence.
In 1905, Einstein further promoted Planck's concept of quantization.
He pointed out that not only the energy exchange between the black body and the radiation field is quantized, but also the radiation field itself is composed of discontinuous light quanta, and the relationship between the energy of each light quantum and the frequency of the radiation field satisfies ε=hν , that is, its energy is only related to the frequency of the light quantum, and has nothing to do with the intensity (amplitude).
According to Einstein's light quantum theory, the light emitted to the metal surface is essentially a stream of photons with energy ε=hν.
If the frequency of the irradiating light is too low, that is, the energy of each photon in the photon stream is small, when it irradiates the metal surface, the electron absorbs this photon, and the energy it adds is ε=hν It is still less than the work function required for electrons to leave the metal surface, and the electrons cannot leave the metal surface, so the photoelectric effect cannot be produced.
If the frequency of the irradiated light is high enough to cause the absorbed electrons to have enough energy to overcome the work function and break away from the metal surface, the photoelectric effect will occur. At this time, the relationship between the kinetic energy of the emitted electron, the photon energy and the 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: Εk(max)=hv-W0, this is Einstein’s photoelectric effect equation.
Baidu Encyclopedia: Photoelectric Effect