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Coherence And Coherent Sources: Definition, Types & Examples

Coherence And Coherent Sources: Definition, Types & Examples

Have you ever seen laser light? And do you know what makes it different from the normal light? Lasers generate coherent light, which sets them apart from any other source of light. Coherence and coherent sources form the basis of some commercial applications like the Sagnac Gyroscope, optical coherence tomography, holography, and telescope interferometers.

Table of contents

  • What is Coherence?
  • Coherent Sources
  • Examples of Coherence and Coherent Sources
  • Types of Coherence
  • Production of Coherent Sources
  • Applications of Coherence and Coherent Sources
  • Practice Problems
  • FAQs

What is Coherence?

The ability of waves to retain a stable connection with one another through time and place is referred to as coherence. It explains the relationship between waves and establishes the interference patterns that occur. In contrast to incoherent waves, which display unpredictable phase relationships, coherent waves have a constant phase connection, resulting in constructive interference.

Coherent & Non-Coherent Waves

Image: Coherent & Non-Coherent Waves

Coherent Sources

Waves produced by coherent sources have a defined phase relationship. These resources are crucial for many applications in science and technology. Lasers, synchrotron radiation sources, coherent neutron sources, and coherent sound sources are a few examples of coherent sources.

Examples of Coherence and Coherent Sources

  1. Lasers: The most well-known coherent source is probably the laser. They discharge powerful, tightly focused light waves that have a defined phase connection. Due to their coherence, lasers may produce interference patterns, crisp diffraction patterns, and exact measurements for use in materials processing, medicine, and telecommunications.
  1. Synchrotron Radiation Sources: Very coherent electromagnetic waves, including X-rays, are produced by synchrotron radiation sources. Synchrotrons produce coherent beams by rapidly accelerating charged particles, such as electrons, and then forcing them to emit radiation. Researchers can conduct tests in a variety of disciplines, such as materials science, biology, and chemistry, thanks to its outstanding coherence.
  1. Coherent Neutron Sources: Coherent neutron beams are produced by specialist equipment like research reactors and spallation sources. Eutrons can also show this property. Condensed matter physics, materials engineering, and archaeology may all benefit from investigating the structure and behaviour of materials at the atomic level with the help of coherent neutron sources.
  1. Coherent Sound Sources: Coherent sound sources are employed in acoustics for many different purposes. For instance, the sound waves produced by musical instruments have set phase relationships, which enable the production of melodic tones. Coherent sound sources are also utilised in sonar and ultrasonic imaging systems, where exact control over the phase relationships of the output sound waves is necessary for accurate measurements and imaging.

Types of Coherence

  1. Temporal Coherence: The connection between waves at various times in space and time is referred to as temporal coherence. For an extended period, waves with great temporal coherence keep their phase connection steady. A temporally coherent source includes laser light.
  1. Spatial Coherence: The correlation between waves at various locations in space is referred to as spatial coherence. High spatial coherence waves keep their phase connection constant across long distances. A type of spatially coherent wave is the light that a far-off star emits.
  1. Longitudinal Coherence: Waves that are moving in a single direction fall under the definition of longitudinal coherence. It talks about the relationships between waves at various locations along this trajectory. Sound waves display longitudinal coherence while passing through a pipe.

Production of Coherent Sources

The particular kind and application determine whether coherence and coherent sources are produced. For instance, although synchrotron radiation sources rely on charged particle acceleration, lasers produce coherence by stimulated emission in a gain medium. The production techniques need exact engineering and control since they are so specialised.

Applications of Coherence and Coherent Sources 

The applications for coherence and coherent sources may be found in many domains, such as:

  • Microscopy and optical imaging
  • Measurements with precision using interferometry
  • 3D imaging and holography
  • processing quantum information
  • Analysing materials and using X-rays
  • Imaging and non-destructive testing methods

Practice Problems

Q1. Which of the following options is an example of a coherent source?

a) Incandescent light bulb
b) Fluorescent lamp
c) Laser
d) Candle flame

Answer: (c) Laser

Explanation: Laser is the only example of coherent sources from the above 4 options. The rest of the options, like bulb, lamp, and candle flame, can not be considered coherent sources.

Q2. What scientific phrase best describes the relationship between waves at various moments in time?

a) Temporal coherence
b) Spatial coherence
c) Longitudinal coherence
d) Transverse coherence

Answer: (a)  Temporal coherence

Explanation: Temporal coherence is the term that is used to describe the relation of waves at various points in time and space.

Q3. In which of the following phenomena is coherence crucial?

a) Diffraction
b) Polarization
c) Doppler effect
d) Photoelectric effect

Answer: (a) Diffraction

Explanation: Diffraction is an important phenomenon that lays the foundation of coherence. It is there crucial for coherence.


Q1. Why is coherence important in wave phenomena?
Since it controls how interference and diffraction patterns behave, coherence and coherent sources are significant in wave phenomena. This characteristic enables accurate measurements, imaging, and wave behaviour research. It would be challenging to see and study wave phenomena if there were no coherence because waves would have unpredictable phase relationships and cause destructive interference.

Q2. Can incoherent sources produce interference patterns?
Answer: In general, incoherent sources do not result in clearly defined interference patterns. The interference that results from incoherent sources' random phase connections is unexpected but eventually tends to average out.

Q3. How does coherence affect the quality of optical imaging?
Answer: An important factor in optical imaging is coherence. Beams that are extremely focused and clearly defined are produced by coherent light sources, such as lasers. Coherent light maintains its phase connection as it travels through an optical system, producing exact interference and diffraction patterns.

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