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What is Coherence?

To understand coherence physics, let us first take a look at the concept of coherent light wave. When two or more light waves are traveling together in such a way that their phase difference with respect to time is constant, then such waves are known as the coherent waves. The property of any light source to measure a definite correlation between any two points with the same frequency and amplitude is known as the coherence of light and the light source is known as the coherent light source. 

LASER light is considered to be the best example of the coherent light source. There are no perfect coherent sources. The laser is a highly coherent light source.


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Now let us have a look at coherence physics or coherent meaning in physics. In physics when we are discussing concepts of optics, the behavior of light plays an important role. One of the important parameters associated with the brightness of any light source is its coherence ability. Now, what is coherence? The coherence of light gives the correlation between any two points of light waves which have a constant phase with respect to time is known as the coherence of light, this property is known as coherence and this is the coherence definition Physics. There are two types of coherence depending on the position and time and are given by:

  1. Temporal coherence

  2. Spatial coherence

Types of Coherence

For understanding coherent light meaning with further details, let us start with the discussion of types of coherence in detail.

Temporal Coherence

  • It is also known as longitudinal coherence. It is a type of coherence depending upon the time.

  • It is a measure of the correlation of light waves phase at different directions of propagation.

  • The temporal coherence has explained the monochromaticity of the light source.

  • The temporal coherence is explained by the lifetime of atoms in their excited state.

  • This property of light is observed when atoms de-excite from their excited state simultaneously. For this simultaneous de-excitation, the lifetime of atoms must be as large as possible.

  • The average time period for which the field remains sinusoidal is known as the coherence time or the temporal time. The average length for which the waveforms remain sinusoidal is known as the coherence length.


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Spatial Coherence

  • It is a type of coherence depending upon the position.

  • It is a measure of the correlation of the phase of the light wave at different transverse to the direction of propagation.

  • The spatial coherence has explained the finite size of the light source.

  • The ideal point source exhibits perfectly spatial coherence, whereas extended point sources are bound to exhibit less spatial coherence.


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Achieving absolute coherence is impossible, rather we can get partial coherence or the degree of coherence in nature. 


Incoherence and Incoherent Sources

  • Incoherence refers to the phenomenon where two or more waves considered are out of phase or the correlation between the two points are not in phase with each other. 

  • The property of light sources such that there will be no correlation between the two emitted waves. All-natural and conventional sources of light are incoherent in nature. 

  • Incoherence explains the random motion of the emitted waves.

Did You Know

While studying quantum optics, the term coherence is often used for the state of light-emitting atoms or ions. In that case, coherence refers to a correlation and phase relationships between the complex amplitudes corresponding to electronic states. This is important, e.g., in the context of lasing without inversion. There is also the term coherent states of the light field, which has yet another meaning.

FAQs (Frequently Asked Questions)

1. What is Coherence in Physics?

Ans: The property of any light source to measure a definite correlation between any two points with the same frequency and amplitude is known as the coherence of light and the light source is known as the coherent light source.

2. What are Two Types of Coherence?

Ans: There are two types of coherence depending on the position and time and are given by:

  • Temporal coherence

  • Spatial coherence

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