Have you ever wondered what many light phenomena are actually? For example, take the photoelectric effect, heat waves, photochemical processes and the like - all these are the quantum properties of light. If they were not open, the works of scientists would not have gone off the ground, in fact, like scientific and technological progress. They are studied in the section of quantum optics, which is inextricably linked with the same section of physics.
Quantum Properties of Light: Definition of the Term
Until recently, a clear and understandable interpretation of this optical phenomenon could not be given. It was successfully used in science and everyday life, on its basis were built not only formulas, but also entire tasks in physics. Only the modern scientists who summed up the activities of their predecessors succeeded in formulating the final definition. So, the wave and quantum properties of light are a consequence of the characteristics of its emitters, which are the electrons of atoms. A quantum (or photon) is formed due to the fact that the electron goes to a lower energy level, thereby generating electromagnetic pulses.
The first optical observations
The assumption of the presence of quantum properties in light appeared in the 19th century. Scientists have discovered and diligently studied phenomena such as diffraction, interference, and polarization. With their help, the electromagnetic wave theory of light was derived. It was based on the acceleration of the motion of electrons during the oscillation of the body. Due to this, heating occurred, and light waves appeared after it. The first author's hypothesis on this subject was formed by the Englishman D. Rayleigh. He regarded radiation as a system of identical and constant waves, moreover, in a confined space. According to his findings, with a decrease in the wavelength, their power should continuously increase, moreover, the presence of ultraviolet and X-ray waves was required. In practice, all this was not confirmed, and another theorist took up the matter.
Planck formula
At the very beginning of the 20th century, Max Planck, a physicist of German origin , put forward an interesting hypothesis. According to her, the emission and absorption of light does not occur continuously, as previously thought, but in portions - by quanta, or, as they are also called, by photons. The Planck constant was introduced - the proportionality coefficient denoted by the letter h , and it was equal to 6.63 · 10 -34 J · s. In order to calculate the energy of each photon, one more quantity was required - v - the frequency of light. The Planck constant was multiplied by the frequency, and as a result received the energy of a single photon. Thus, a German scientist accurately and correctly consolidated in one simple formula the quantum properties of light, which had previously been discovered by G. Hertz and designated by him as a photoelectric effect.
Opening photo effect
As we have already said, the scientist Heinrich Hertz was the first to draw attention to the previously unnoticed quantum properties of light. The photoelectric effect was discovered in 1887, when a scientist combined an illuminated zinc plate and an electrometer rod. If a positive charge reaches the plate, the electrometer does not discharge. If a negative charge is emitted, the device begins to discharge as soon as an ultraviolet ray hits the plate. In the course of this practical experience, it was proved that a plate under the influence of light can emit negative electric charges, which later received the corresponding name - electrons.
Practical experiments of Stoletov
Practical experiments with electrons were conducted by Russian researcher Alexander Stoletov. For his experiments, he used a vacuum glass bottle and two electrodes. One electrode was used to transfer energy, and the second was illuminated, and the negative pole of the battery was connected to it. During this operation, the amperage began to increase, but after a while it became constant and directly proportional to the radiation of the light flux. As a result, it was found that the kinetic energy, as well as the delaying voltage of the electrons, are independent of the power of light radiation. But an increase in the frequency of light makes this indicator grow.
New quantum properties of light: the photoelectric effect and its laws
In the course of the development of Hertz's theory and Stoletov’s practice, three basic laws were derived, according to which, as it turned out, photons function:
1. The power of light radiation that falls on the surface of the body is directly proportional to the strength of the saturation current.
2. The power of light radiation does not affect the kinetic energy of photoelectrons, but the frequency of light is the cause of the linear growth of the latter.
3. There is a certain “red border of the photoelectric effect”. The bottom line is that if the frequency is less than the minimum indicator of the frequency of light for a given substance, then the photoelectric effect is not observed.
The difficulties of the collision of the two theories
After the formula deduced by Max Planck, science faced a dilemma. The previously derived wave and quantum properties of light, which were discovered a little later, could not exist within the framework of generally accepted physical laws. In accordance with the electromagnetic, old theory, all the electrons of the body on which the light hits, must come into forced oscillation at equal frequencies. This would generate infinitely large kinetic energy, which is impossible. Moreover, for the accumulation of the necessary amount of energy, the electrons had to remain at rest for tens of minutes, while the photoelectric effect was observed in practice without any delay. Additional confusion also arose due to the fact that the energy of photoelectrons was independent of the power of light radiation. In addition, the red border of the photoelectric effect has not yet been discovered, and the proportionality of the frequency of light to the kinetic energy of electrons has not been calculated. The old theory could not clearly explain the physical phenomena visible to the eye, and the new one was not yet fully developed.
Rationalism of Albert Einstein
Only in 1905 did the brilliant physicist A. Einstein identify in practice and clearly formulate in theory what it is - the true nature of light. The wave and quantum properties discovered with the help of two opposite hypotheses are inherent to photons in equal parts. For completeness, the only thing missing was the principle of discreteness, that is, the exact location of quanta in space. Each quantum is a particle that can be absorbed or radiated as a whole. An electron, "swallowing" a photon inside itself, increases its charge by the value of the energy of the absorbed particle. Further, inside the photocathode, the electron moves to its surface, while maintaining a “double portion” of energy, which at the output is converted into kinetic. In such a simple way, the photoelectric effect is realized in which there is no belated reaction. At the very finish, an electron releases a quantum that falls on the surface of the body, emitting even more energy. The larger the number of emitted photons, the more powerful the radiation, respectively, and the oscillation of the light wave increases.
The simplest devices based on the principle of the photoelectric effect
After discoveries made by German scientists at the dawn of the twentieth century, began the active use of the quantum properties of light for the manufacture of various devices. Inventions, the principle of which is the photoelectric effect, are called photocells, the simplest representative of which is vacuum. Among its shortcomings can be called weak current conductivity, low sensitivity to long-wave radiation, which is why it cannot be used in AC circuits. The vacuum device is widely used in photometry, they measure the strength of brightness and quality of light. It also plays an important role in photo telephones and in the process of sound reproduction.
Conductive photocells
This is a completely different type of devices, which are based on the quantum properties of light. Their purpose is to change the concentration of current carriers. This phenomenon is sometimes called the internal photoelectric effect, and it forms the basis of the work of photoresistors. These semiconductors play a very important role in our daily lives. They were first used in retro cars. Then they provided the work of electronics and batteries. In the mid-twentieth century, similar solar cells began to be used for the construction of spacecraft. Until now, due to the internal photoeffect, turnstiles in the subway, portable calculators and solar panels work.
Photochemical reactions
Light, the nature of which only became partially accessible to science in the 20th century, actually affects chemical and biological processes. Under the influence of quantum flows, the process of dissociation of molecules and their fusion with atoms begins. In science, this phenomenon is called photochemistry, and in nature, one of its manifestations is photosynthesis. It is due to light waves in the cells that processes are carried out to release certain substances into the intercellular space, due to which the plant acquires a green tint.
The quantum properties of light also affect human vision. Getting on the retina of the eye, the photon provokes the process of decomposition of the protein molecule. This information is transported by neurons to the brain, and after processing it, we can see everything in the light. With the onset of darkness, the protein molecule is restored, and vision is accommodated to new conditions.
Summary
In the course of this article, we found out that mainly the quantum properties of light manifest themselves in a phenomenon called the photoelectric effect. Each photon has its own charge and mass, and when it collides with an electron, it gets inside it. Quantum and electron become one, and their combined energy turns into kinetic energy, which, in fact, is required for the implementation of the photoelectric effect. In this case, wave oscillations can increase the energy produced by a photon, but only up to a certain indicator.
The photoelectric effect today is an indispensable component of most types of equipment. Space liners and satellites are built on its basis, solar panels are developed, and used as a source of auxiliary energy. In addition, light waves have a huge impact on the chemical and biological processes on Earth. Due to simple sunlight, the plants turn green, the earth's atmosphere turns blue over the whole palette, and we see the world as it is.