الظاهرة الكهرضوئية - الضوء .. الفيزياء

Photoelectric effect - Light .. Physics

Photoelectric effect

In the late nineteenth century, scientists saw that exposing a clean zinc plate to light or ultraviolet rays gives it a positive electrical charge. Since the number of electrons in a metal is usually equal to the number of its protons, this means that the rays caused the metal to lose a number of electrons.

The classical theory of waves tells us that the energy of the wave is related only to its intensity and is never related to the value of its frequency, and that the value of the energy exchanged between the rays and the electron on the surface of the metal increases as the intensity of the wave forming these rays increases. It has been scientifically proven that increasing the intensity of light had no real effect on the energy of the emitted electrons, but rather helped to increase their emitted numbers. It has also been proven that raising the value of the energy of the rays can only be done by raising the value of their frequency.

However, quantum theory was able to provide an explanation for this phenomenon, which appears to be strange on the surface. If we consider that the rays are a vapor of photons with an energy proportional to the frequency of the rays themselves, then any increase in the frequency of the rays increases the value of the energy exchanged between the photons and electrons, and the stronger the intensity of the light, the greater the number of photons and consequently the number of electrons that caused their release.

There are many applications of the photoelectric theory. When photoelectrons are released, they are attracted towards a positive anode, then pass through an electronic circuit, forming an electric current whose intensity is linked to the intensity of the rays themselves. The device that uses this principle is called a photoelectric cell and is used as a meter. • In measuring the intensity of both light and ultraviolet rays, the exposure used in photography is based on the effect of electric light on a plate of selenium or cadmium sulfate.

It is known that photons are not able to slow down their speed, they completely disappear in the photoelectric phenomenon and they differ from other particles of matter such as electrons, which are unable to reach the speed of light.
It is worth noting that the theory of special relativity predicted that the mass of a particle K differs from its mass in the state of stasis K5 and that it increases with the increase of its speed V according to the following equation K = K5 / root 1 - V2 / H2. Therefore, the mass of the stagnant state is zero in the photon that usually travels at the speed of light, otherwise it becomes an infinite mass in the case of its movement. The relationship between the photoelectric effect and the energy of the electrons is determined by the frequency of the incident light, while the number of emitted electrons is related to the intensity of the light.