Coherent Source and Incoherent Source

We know light is an essential part of living and is classified based on wavelength, intensity, uses, sources, colour and frequencies. Light waves are composed of photons and the waves are characterised by various features like phase, amplitude, frequency and period. The light produced by various sources is of different colours depending on the wavelength and frequency.

Depending on the source of light produced, they are classified as

  1. Coherent sources
  2. Incoherent sources

The interrelation between physical quantities of a single wave or between several waves is described by the property known as coherence. Let us know in detail about the coherent source and Incoherent source.

Coherent Source

The source which emits a light wave with the same frequency, wavelength and phase or having a constant phase difference is known as a coherent source. A coherent source forms sustained interference patterns when the waves superimpose and the positions of maxima and minima are fixed.

Coherent sources produce waves that have a constant phase difference with the same frequency and amplitude. The light from a LASER is coherent, parallel, monochromatic and has unbroken wave chains. The pattern of the coherent wave is as shown in the picture below.

The coherent source is produced by using prisms, lenses and mirrors with specific specifications. Some of the techniques that help in the production of coherent sources are Fresnel’s biprism, Young’s double-slit experiment and Lloyd’s mirror arrangement. In these techniques, the division of the wavefront takes place to produce a coherent source.

Considering the incoming beam and dividing its amplitude into various parts through a process of partial reflection or refraction aids in the creation of coherent sources. These divided beams travel in different paths and meet other waves and undergo interference, where the division in amplitude takes place to create a coherent source.

Examples of Coherent Sources

  • A LASER is a type of coherent source. LASER uses a phenomenon known as stimulated emission to generate highly coherent light.
  • Sound waves produced from speakers are driven by electrical signals that have the same frequency and a definite phase.

Incoherent Source

Conventional light sources are used to produce incoherent sources. An incoherent source emits a light wave having a different frequency, wavelength and phase. In these waves, the transitions between energy levels in an atom is a completely random process. Hence, no control can be established over an atom that is going to lose energy in the form of radiation. The pattern of the incoherent wave is as shown in the picture below.

Examples of Incoherent Sources

Tungsten filament lamps aid in the production of the incoherent source. Normal fluorescent tubes emit incoherent light.

Read more about the reflection of the light.

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Frequently Asked Questions on Coherent Source and Incoherent Source

1. What type of light is LASER?

LASER is a monochromatic light.

2. What are the types of light interference?

  • Constructive interference
  • Destructive interference

3. Do coherent sources produce waves of constant phase difference?


4. Where are coherent superpositions of optical wave fields used?


5. Tungsten filament lamps produce which type of light?

Incoherent source of light.

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