Coherence (Physics): Definition & Meaning

In this article we explain what is meant by coherence, when interference phenomena can occur and what distinctions there are.

Coherence is a sub-area of ​​waves and belongs to the subject of physics.

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Coherence is not only used in physics, but also in psychology and pedagogy, among other things.

What is coherence?

In physics, coherence describes the property of waves to follow a common fixed rule in the dynamic course. The term derives from the Latin word «cohaerere» and means «to connect».

In general, one can say that coherence denotes a defined phase relationship between states of different energies or between several quasiparticles.

Coherence is the property of two wave trains that exists when their phase shift at one location either remains constant for all time or when it changes lawfully over time. In addition, the coherence is defined as the totality of all correlation properties between wave quantities.

All physical waves such as light waves, radar waves, sound waves or water waves can be coherent with other waves in a certain way. In addition, it is possible that there is only coherence between corresponding partial waves.

The presence of coherence often indicates a common origin history of the waves. So if the same causal mechanism was the basis for wave generation, constant oscillation patterns can arise in the wave train. In a later comparison of partial waves, these oscillation patterns can be made visible.

For example, while it is possible to keep sound waves coherent for a practically unlimited time, the emission of light only emits more or less long, coherent wave trains. Two different wave trains are incoherent and therefore cannot interfere with each other. Interference only occurs with light if a wave train is divided at a beam splitter, for example, and then recombined at another point.

Periodic waves are simple cases. With them, two partial waves are coherent if there is a fixed phase relationship to one another. In optics, this phase relationship often means a constant difference between the phases of the oscillation period.

  • Coherence describes a defined phase relationship between states of different energies or between several quasiparticles.
  • Coherence is the property of two wave trains that exists when their phase shift at one location either remains constant for all time or when it changes lawfully over time.
  • The presence of coherence often indicates a common origin history of the waves.
  • It is possible that coherence only exists with partial waves.
  • In the case of periodic waves, two partial waves are coherent if there is a fixed phase relationship to one another.

Interference phenomena due to coherence

A particularly clear form of coherence occurs when stationary interference phenomena become visible when waves are superimposed.

Interference phenomena are phenomena caused by the interference of light, which can occur in very different ways. Depending on the type, shape, number and mutual position of the optical media involved, there are spots, regular figures, stripes, rings, curves or others. The interference phenomena are explained in more detail in the article on interference.

The occurrence of stationary, spatially and temporally unchanging interference phenomena when the waves are superimposed shows that the wave amplitudes of two waves correlate directly with one another. If no interference phenomena are visible, a higher level of technical effort or a more complicated mathematical analysis of the wave profile is required to demonstrate coherence in the waves.

  • If interference phenomena become visible when waves are superimposed, then there is a very clear type of coherence.
  • Interference phenomena show that the wave amplitudes of two waves are directly correlated.
  • Interference phenomena are wave-typical phenomena that can occur in a wide variety of forms.

What types of coherence are there?

The concept of coherence can be further subdivided and specified into different categories. These are now described below.

What is meant by complete and partial coherence?

There is complete coherence, partial coherence and incoherence. Partial waves that are superimposed at a fixed location to a specific time-averaged intensity can be amplified or extinguished depending on the phase relationship. When that happens, there is complete coherence. In the case of partial coherence, they weaken only slightly or only slightly strengthen.

The incoherence

If the partial waves equalize to an average intensity, this is called incoherence. The phenomenon thus means the absence of a defined phase relationship. In particular, there is incoherence at different frequencies when all phase differences occur with the same frequency and no constructive or destructive interference is possible as a result.

Spatial and temporal coherence

What can also be distinguished within coherence is spatial and temporal coherence. Normally, however, both forms of coherence must be present. Temporal coherence is when there is a fixed phase difference along the time axis. The coherence time or the coherence length of a light wave can be measured by splitting these two partial beams and later combining them again. This is possible with certain interferometers.

Interferometers are devices that can be used to determine differences in length very precisely. The wavelength of the light is used as a unit of measurement by determining the path difference in two paths that are to be compared.

One sees interference phenomena in a certain arrangement only if the difference in transit time or the difference in path between the partial waves remains smaller than the coherence time or the coherence length of the wave trains emitted by the atoms.

Spatial coherence, on the other hand, is present when there is a fixed phase difference along a spatial axis. Similar to temporal coherence, spatial coherence can be determined by measuring an interferometer that is sensitive to spatial coherence. Here the contrast of an interference pattern is examined.

Waves with different frequencies can also be coherent with one another. Technically, this type of coherence plays a role in radar technology or in the frequency comb.

  • Complete coherence occurs when partial waves that are superimposed at a fixed location to a specific time-averaged intensity are amplified or canceled depending on the phase relationship.
  • With partial coherence, the same thing happens as with full coherence, but in a weaker form.
  • Incoherence is the absence of a defined phase relationship.
  • Temporal coherence is when there is a fixed phase difference along the time axis.
  • Spatial coherence, on the other hand, exists when there is a fixed phase difference along a spatial axis.

In which areas of physics does coherence play a role?

Coherence plays a role in all areas of physics where interference can be observed. Coherence is an important component, particularly in laser optics, spectroscopy and interferometry. It does not play a role for the meaning of coherence whether it is about light waves or matter waves.

Above all in laser technology, it is possible to produce numerous copies of individual photons with a coherent history of their origin. For this reason, coherence is also of great importance in the application areas of laser technology. For example, coherence is important in the creation of holograms, in quantum cryptography or in signal processing.

  • Coherence plays a role in all areas of physics where interference is observed.
  • Coherence is particularly important for laser optics, spectroscopy and interferometry.
  • Areas of application of laser technology, such as the creation of holograms, benefit from coherence.

Everything you need to know about coherence at a glance

  • In physics, coherence describes the property of waves to follow a common fixed rule in the dynamic course.
  • Coherence is defined as the total of all correlation properties between wave magnitudes.
  • All physical waves can be coherent with other waves in some way. In addition, there can be coherence between certain partial waves.
  • The presence of coherence often indicates a common origin history of the waves.
  • A particularly clear form of coherence occurs when stationary interference phenomena become visible when waves are superimposed.
  • There is complete coherence, partial coherence and incoherence.
  • A distinction is also made between spatial and temporal coherence, with both forms usually being present at the same time.
  • Coherence plays a role in all areas of physics where interference can be observed, such as in laser optics, spectroscopy, and interferometry.