This article considers such a phenomenon of physics as interference: what is it, when it arises and how it is applied. Also described in detail about the related concept of wave physics - diffraction.
Types of waves
When the word “wave” appears in a book or in a conversation, then, as a rule, the sea immediately appears: the blue expanse, the immense distance, one after the other, salty ramparts run ashore. A resident of the steppes will imagine a different view: the vast expanse of grass, it sways in the gentle breeze. Someone else will remember the waves, looking at the folds of a heavy curtain or the flutter of a flag on a sunny day. A mathematician will think about a sinusoid, a radio lover will think about electromagnetic oscillations. All of them have a different nature and belong to different species. But one thing is indisputable: a wave is a state of deviation from equilibrium, the transformation of some kind of “smooth” law into an oscillatory one. It is for them that the phenomenon of interference is applicable. What is and how it arises, we will consider a little later. First, let's see what waves are. We list the following types:
- mechanical;
- chemical;
- electromagnetic;
- gravitational;
- spin;
- probabilistic.
From the point of view of physics, waves carry energy. But it happens that the mass moves. Answering the question of what interference is in physics, it should be noted that it is characteristic of waves of absolutely any nature.
Signs of wave difference
Oddly enough, but there is no single definition of a wave. Their species are so diverse that there are more than a dozen types of classification. What are the signs of distinguishing waves?
- By the method of distribution in the environment (running or standing).
- By the nature of the wave itself (vibrational and solitons are different precisely by this feature).
- By type of distribution in the medium (longitudinal, transverse).
- By degree of linearity (linear or non-linear).
- By the properties of the medium in which they are distributed (discrete, continuous).
- In shape (flat, spherical, spiral).
- According to the characteristics of the physical propagation medium (mechanical, electromagnetic, gravitational).
- In the direction of oscillation of the particles of the medium (compression or shear waves).
- By the time required to excite the medium (single, monochromatic, wave packet).
And interference is applicable to any type of these disturbances of the medium. What is special is contained in this concept and why exactly this phenomenon makes our world the way it is, we will tell you after the characteristics of the wave have been reduced.
Wave characteristics
Regardless of the type and type of waves, they all have common characteristics. Here is the list:
- A comb is a kind of maximum. For compression waves, this is the place with the highest density of the medium. Represents the largest positive deviation of the oscillation from the equilibrium state.
- Lozhbina (in some cases, a valley) is a concept inverse to the crest. Minimum, largest negative deviation from equilibrium.
- The temporal periodicity, or frequency, is the time during which the wave passes from one maximum to another.
- Spatial periodicity, or wavelength, is the distance between adjacent peaks.
- Amplitude is the height of the peaks. It is this definition that will be needed to understand what wave interference is.
We examined in detail the wave, its characteristics and various classifications, because the concept of “interference” cannot be explained without a clear understanding of such a phenomenon as disturbance of the medium. We remind you that interference makes sense only for waves.
Wave interaction
Now we have come close to the concept of “interference”: what is it, when does it arise and how to determine it. All the above types, types and characteristics of waves belonged to an ideal case. These were descriptions of the "spherical horse in a vacuum," that is, some theoretical constructions impossible in the real world. But in practice, the entire space around is riddled with various waves. Light, sound, heat, radio, chemical processes are periodic fluctuations of the medium. And all these waves interact. One feature should be noted: so that they can influence each other, they must have similar characteristics.
Sound waves can in no way interfere with light, and radio waves do not interact with the wind in any way. Of course, the influence is still there, but it is so small that its action is simply not taken into account. In other words, when explaining what the interference of light is, it is assumed that one photon affects another when it meets. So, more details.
Interference
For many types of waves, the principle of superposition applies: when they meet at one point in space, they interact. The exchange of energy is displayed on the change in amplitude. The law of interaction is as follows: if two maximums meet at one point, then in the final wave the maximum intensity doubles; if a maximum and a minimum are encountered, then the total amplitude becomes zero. This is a clear answer to the question of what is the interference of light and sound. In fact, this is an overlay phenomenon.
Wave interference with different characteristics
The event described above represents a meeting of two identical waves in linear space. However, two counterpropagating waves can have different frequencies, amplitudes, and lengths. How to present the final picture in this case? The answer lies in the fact that the result will not be quite like a wave. That is, the strict order of alternating highs and lows will be violated: at some point, the amplitude will be maximum, at the next - already less, then a maximum and a minimum will meet and the result will turn to zero. However, no matter how strong the differences between the two waves, the amplitude will still happen sooner or later. In mathematics, it is customary to talk about infinity, but in reality, the forces of friction and inertia can stop the very existence of the resulting wave before the picture of peaks, valleys and plains repeats itself.
Interference of waves meeting at an angle
But, in addition to its own characteristics, the position in space may differ in real waves. For example, when considering what the interference of sound is, this must be taken into account. Imagine: a boy is walking and blowing a whistle. He sends a sound wave in front of him. And another boy rides past him on a bicycle and rings the bell so that the pedestrian is outside. At the meeting point of these two sound waves, they intersect at a certain angle. How to calculate the amplitude and shape of the final oscillation of the air, which will reach, for example, the nearest trader of granny Masha's seeds? Here the vector component of the sound wave comes into effect. And in this case it is necessary to add or subtract not only the magnitude of the amplitude, but also the propagation vectors of these oscillations. We hope that Grandma Masha will not scream much at the noisy guys.
Interference of light with different polarization
It also happens that at one point there are photons of different polarization. In this case, the vector component of electromagnetic waves should also be taken into account. If they are not mutually perpendicular or one of the light beams has circular or elliptical polarization, then the interaction is quite possible. Several methods for determining the optical purity of crystals are built on this principle: there should be no interaction in perpendicularly polarized beams. If the picture is distorted, then the crystal is imperfect, it changes the polarization of the beams, which means that it is grown incorrectly.
Interference and diffraction
The interaction of two light beams leads to their interference, as a result, the observer sees a series of light (maxima) and dark (minima) bands or rings. But the interaction of light and matter is accompanied by another phenomenon - diffraction. It is based on the fact that light of different wavelengths is otherwise refracted by the medium. For example, if the wavelength is 300 nanometers, then the deviation angle is 10 degrees, and if 500 nanometers is already 12. Thus, when light from a ray of sunlight hits a prism from quartz, red refracts differently from violet (their wavelengths differ) , and the observer sees the rainbow. This is the answer to the question of what is the interference and diffraction of light and how they differ. If monochromatic radiation from a laser is directed to the same prism, there will be no rainbow, since there are no photons of different wavelengths. It’s just that the beam deviates from the original direction of propagation by a certain angle, and that’s all.
Putting the phenomenon of interference into practice
There are many opportunities to get practical benefits from this purely theoretical phenomenon. Only the main ones will be listed here:
- Crystal quality research. We talked about this a little higher.
- Identification of lens errors. Often they must be ground in perfect spherical shape. The presence of any defects is detected precisely by the phenomenon of interference.
- Determination of film thickness. In some types of production, constant film thickness, such as plastic, means a lot. It is the phenomenon of interference together with diffraction that makes it possible to determine its quality.
- Enlightenment of optics. Glasses, lenses of cameras and microscopes are covered with a thin film. Thus, electromagnetic waves of a certain length are simply reflected and superimposed on themselves, reducing interference. Most often, enlightenment is done in the green part of the optical spectrum, since it is this area that the human eye perceives best.
- Space exploration. Knowing the laws of interference, astronomers are able to separate the spectra of two closely spaced stars and determine their compositions and distance to the Earth.
- Theoretical research. It was once with the help of the phenomenon of interference that it was possible to prove the wave nature of elementary particles, such as electrons and protons. This confirmed the hypothesis of the particle-wave duality of the microworld and laid the foundation for the quantum era.
We hope that with this article, your knowledge about the application of coherent (emitted by sources having a constant phase difference and the same frequency) waves has expanded significantly. This phenomenon is called interference.