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Coherence is a fixed relationship between the phase of waves in a single frequency beam of radiation. They are coherent when the phase difference between two light beams is consistent; they are noncoherent when the phase difference is random or fluctuating. Only coherent sources emit stable interference patterns, typically created by splitting a single beam into two or more beams. Unlike an incandescent source, a laser produces a beam in which all components are in a fixed relationship.
Coherent Source
The two sources are junctions if they have zero or continuous phase differences with the same amount. Most light sources in the environment, such as a bulb, the sun, a candle, and so on, are made up of numerous disconnected light sources. The laser is a model with parallel sources, meaning that numerous critical sources inside the laser are locked in stages. When two coherent light sources have the same frequency and phase difference, they are said to be coherent.
Coherence is a property of waves that allows for stationary interference. In addition, interference visibility is frequently employed to estimate coherence.
Two equal slits illuminated by a laser beam can be considered two parallel point sources.
Incoherent Sources
Incoherent sources are the polar opposite of coherent sources. These sources produce light with a fixed frequency and random phase variations between photons.
Incoherent sources include all traditional light sources. Transitioning between the energy levels of an atom is a fully random operation. As a result, we cannot know when an atom will lose energy as radiation.
For example: electric bulb, night lamp, etc.
Methods of Producing Coherent Source
There are two different methods of producing coherent sources:
By splitting the wavefront
There are several sections to the wavefront. We can use a variety of lenses, mirrors, and even prisms to achieve our goals. Procedures such as the double-slit experiment, by Thomas Young, biprism approach by Fresnel, and others utilise this method.
Division of the amplitude
It is created by separating the amplitude of an entering beam into distinct pieces through incomplete reflection/refraction. These components later collide, causing interference.
It is possible to use the Newton ring phenomenon or Michelson’s interferometer arrangement or configuration.
Characteristics of Coherent Sources
The characteristics of coherent sources are listed below:
The generated waves are either in phase with each other or have a constant phase difference.
All of the waves have the same frequency.
The wave’s amplitudes should be the same.
Examples of coherent sources
A coherent source of light is laser light, for example. The phase of the light emitted by the laser is similar.
Another example of a coherent source is sound waves. Sound waves produce electrical impulses with a comparable frequency and phase.
Interference:
When two light waves from different coherent sources are combined, the first wave’s energy distribution is disrupted by the second. Interference of light is a change in light energy distribution caused by the superposition of two light waves.
When numerous wave disturbances pass through a specific place in a medium, the resultant disturbance at that point is the vector sum of the disturbances created by the individual waves, according to the wave superposition principle.
Types of Coherent Sources
Coherent sources are classified into two parts:
i) Temporal coherence
The average relationship between the value of a wave and itself at any important pair of times is known as temporal coherence.
The temporal coherence is used to evaluate how monochromatic it is. It depicts the ability of a wave to interfere with itself later. The coherence time is defined as the length of time by which the phase or amplitude should be delayed.
ii) Spatial coherence
Consider optical or water waves, where the wave’s dimension varies between one and two spaces.
The capacity of two points in the space of a wave to interfere is thus defined as spatial coherence.
The consistent cross-relationship between two sites in a wave is known as spatial coherence.
The intervention between the range of separation and the two spots can be used to define the diameter of the coherence area.
Conclusion
When two sources have the same frequency and a zero or constant phase difference, they are said to be coherent.
Two independent sources will never be able to work together.
Temporal coherence and spatial coherence are the two types of coherent sources.
Laser beams and X-rays work on the principle of coherence.
Interference occurs when two or more waves collide with the same wavelength and frequency.
Destructive interference occurs when the amplitude of the resultant wave is equal to the difference between the two waves.
