Wave and quantum properties of light
The article reveals the essence of the quantum properties of light. Describes how they were discovered, and what it led to.
Planck and Quant
At the end of the nineteenth and the beginning of the twentieth century, it was believed in scientific circles that everything in physics was completely understandable. The most advanced knowledge at that moment was Maxwell's equations and the study of various phenomena related to electricity. Young people who wanted to pursue science were not recommended to go to physics: after all, there could only be routine research that did not provide any breakthroughs. However, ironically, it was this study of the properties of a long-known phenomenon that opened the way to new horizons of knowledge.
The wave and quantum properties of light began with the discovery of Max Planck. He studied the spectrum of absolute black body and tried to find the most appropriate mathematical description of its radiation. As a result, he came to the conclusion that a certain minimum indivisible quantity should be entered into the equation, which he called the “quantum of action”.And, since it was only a way to “cut the corner” for a simpler mathematical formula, he did not give this value any physical meaning. However, other scientists, for example, A. Einstein and E. Schrödinger, noticed the potential of such a phenomenon as a quantum, and gave rise to a new branch of physics.
I must say that Planck himself did not fully believe in the fundamental nature of his discovery. The scientist, trying to disprove the quantum properties of light, briefly rewrote his formula, starting up various mathematical tricks to get rid of this quantity. But he did not succeed: the genie had already been released from the bottle.
Light - quantum of electromagnetic field
After the discovery of Planck, the already known fact that light has wave properties was supplemented by another: a photon is a quantum of the electromagnetic field. That is, light consists of very small indivisible packets of energy. Each of these packets (photon) is characterized by frequency, wavelength and energy, all of which are related to each other. The speed of light propagation in vacuum is maximum for a known universe and is about three hundred thousand kilometers per second.
It should be noted that quantized (that is, decay into the smallest indivisible parts) and other quantities:
- gluon field;
- gravitational field;
- collective motions of crystal atoms.
Quantum: Unlike Electron
You should not think that in each type of field there is a certain smallest quantity, which is called a quantum: the electromagnetic scale contains both very small and high-energy waves (for example, X-rays), and very large, but “weak” waves (for example, radio waves). ). Just every quantum travels in space as a whole. Photons, it is worth noting, are able to lose some of their energy when interacting with insurmountable potential barriers. This phenomenon is called "tunneling."
The interaction of light and matter
After such a bright discovery, questions sprang up:
- What happens to a quantum of light when it interacts with matter?
- Where does the energy carried by the photon go when it collides with a molecule?
- Why can one wavelength be absorbed and another radiated?
The main thing that has been proved is the pressure of light. This fact gave a new reason for reflection: it means that the photon had impulse and mass.The corpuscular-wave duality of microparticles adopted after this greatly eased the understanding of the insanity that was happening in this world: the results did not fit into any logic that existed before.
Further studies only confirmed the quantum properties of light. The photo effect showed how photon energy is transmitted to matter. Along with reflection and absorption, illumination is able to pull electrons from the surface of the body. How does this happen? The photon transfers its energy to the electron, it becomes more mobile and gains the ability to overcome the force of bonding with the nuclei of matter. The electron leaves its native element and rushes somewhere outside the usual environment.
Types of photoelectric effect
The phenomenon of the photoelectric effect, which confirms the quantum properties of light, has different forms and depends on what kind of solid body the photon encounters. If it collides with a conductor, the electron leaves the substance, as already described above. This is the essence of the external photo effect.
But if a semiconductor or dielectric is illuminated, then the electrons do not leave the limits of the body, but are redistributed, facilitating the movement of charge carriers.Thus, the phenomenon of improving the conductivity during illumination is called an internal photoelectric effect.
Formula external photoelectric effect
Oddly enough, but the internal photo effect is very difficult to understand. It is necessary to know the band field theory, to understand the transitions through the forbidden zone and to understand the essence of the electron-hole conductivity of semiconductors in order to fully realize the importance of this phenomenon. Moreover, the internal photoelectric effect is not so often used in practice. Confirming the quantum properties of light, the formulas of the external photoelectric effect limit the layer from which light can pull electrons.
hν = A + W,
where h is the Planck constant, ν is a quantum of light of a certain wavelength, A is the work that is done by an electron to leave a substance, W is the kinetic energy (and therefore the speed) with which it flies.
Thus, if the entire photon energy is spent only on the electron's exit from the body, then on the surface it will have zero kinetic energy and in fact cannot escape. Thus, the internal photoelectric effect takes place in a rather thin external word of the illuminated substance. This greatly limits its use.
There is a possibility that an optical quantum computer will still use an internal photoelectric effect, but this technology does not yet exist.
The laws of the external photoelectric effect
At the same time, the quantum properties of light are not entirely useless: the photoelectric effect and its laws make it possible to create a source of electrons. Moreover, these laws were formulated in full measure by Einstein (for which he received the Nobel Prize), various prerequisites arose much earlier than the twentieth century. The appearance of a current when electrolyte was illuminated was first observed already at the beginning of the nineteenth century, in 1839.
There are three laws in total:
- The strength of the saturation photocurrent is proportional to the intensity of the light flux.
- The maximum kinetic energy of electrons leaving a substance under the action of photons depends on the frequency (and therefore energy) of the incident radiation, but does not depend on the intensity.
- Each substance with the same type of surface (smooth, convex, rough, nozdrevat) has a red border of the photoelectric effect. That is, there is the smallest energy (and hence the frequency) of the photon, which still separates electrons from the surface.
All these patterns are logical, but they should be considered in more detail.
Explanation of the laws of the photoelectric effect
The first law means the following: the more photons fall per square meter of surface area per second, the more electrons this light can “take” from the substance being illuminated.
An example is basketball: the more often a player throws the ball, the more often he will hit. Of course, if the player is good enough and not injured during the match.
The second law actually gives the frequency response of the outgoing electrons. The frequency and wavelength of a photon determine its energy. In the visible spectrum, red light has the lowest energy. And as many red photons are sent by a lamp to a substance, they are able to transfer only low energy to electrons. Consequently, even if they were pulled out from the surface itself and almost did not complete the work of exiting, their kinetic energy cannot be above a certain threshold. But if we light the same substance with violet rays, then the speed of the fastest electrons will be much higher, even if there are very few violet quanta.
In the third law there are two components - the red border and the state of the surface.Many factors depend on whether a metal is polished or rough, whether there are pores in it, or whether it is smooth or not: how many photons are reflected, how they are redistributed over the surface (obviously, less light gets into the pits). So you can compare with each other different substances only with the same surface condition. But the energy of a photon, which is still able to tear an electron from a substance, depends only on the type of substance. If the nuclei are not very strongly attracted to a charge carrier, then the photon energy may be lower, and, consequently, the red border is deeper. And if the nuclei of a substance hold their electrons firmly and do not want to part with them so easily, then the red border shifts to the green side.