Among the phenomena characteristic of wave processes is the interference of waves, which consists in the fact that during the overlapping of two waves, amplification and weakening of oscillations can occur.
To get acquainted with this phenomenon, we will consider what happens if waves of the same length come to some point in space.
We observe this phenomenon by the example of waves propagating on the surface of the water. We will continuously create waves on its surface at any two points. To do this, in close proximity to the surface of the water, we place the ends of two wires attached to an elastic metal plate. When the plate oscillates, the ends of the wires will periodically immerse in water, and excite vibrations propagating in the form of waves of equal length over its surface. Each of the wires excites its own wave system. Two wave systems, overlapping one another, will interact, which means that there will be interference of waves.
It is important that the two wave systems are consistent, i.e., that at the same length they leave the centers of vibration in the same phases, or if they appear to be phase shifted (for example, arise in opposite phases), then the phase shift with time should not change. Such waves are called coherent. In this experiment, coherence is ensured by the fact that both ends of the wire periodically touch the surface of the water at the same time - the waves leave vibration centers in the same phases.
We construct a picture of the superposition of two systems of coherent waves. The solid circles denote the crests of ring waves propagating along the surface of the water from the sources, and the dashed lines indicate hollows. The points where the waves of both systems meet in the same phases, the trough with the trough and the crest with the crest, are the points of amplified oscillations (maxima). The points of weakened oscillations (minima) are located in places where the depression of one wave meets the crest of another. In the presence of wave coherence, the pattern of alternating maxima and minima will be stable. In fact, if at the moment a crest with a crest is encountered at some point , then after half a period there will also be a hollow with a hollow, and after another half a crest with a crest again, etc., i.e. at this point there will always be a maximum of oscillations. Such a change in the minima and maxima of the oscillation amplitudes is called the interference pattern. And the phenomenon of superposition of waves, forming an interference picture, is called interference of waves.
To solve the question of in which phases the interfering waves will meet at a given point, it is necessary to take into account the difference in their course. To do this, it suffices to calculate the number of waves of the corresponding system that fit at the distances between the points of maxima and minima of interest to us and the centers of oscillations. We will see that the maxima will be at those points that are either at the same distances from both centers, or at the points where the path difference corresponds to an even number of half waves, while the minima to an odd number.
Using the example of this experiment, we examined such a phenomenon as interference of mechanical waves.
Nevertheless, the phenomenon of interference is characteristic not only of waves that occur on the surface of the water, but also of all types of waves: sound, electromagnetic and others. Therefore, if light has wave properties, then when two of its beams are superimposed, interference of light waves will occur.
Sources emitting such waves are called coherent. In optics, coherent sources can only be created artificially.
It should not be forgotten that the interference of waves (any, including light) occurs only when the waves that interact, have the same frequency and time-independent phase shift at any point.