Coherent Sources


Introduction: Coherent Sources of Light

Light sources which produce light waves with the same frequency, constant or zero phases difference, and same wavelength are called coherent sources of light. Two coherent light waves make a sustained interference pattern. Both coherent waves make fixed maxima and minima overtime when they overlap with each other.

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Here you can see this visual to understand coherent sources.

You can read interference and parameters of light to understand this concept elaborately.

Examples of Coherent Sources of Light

Read the following example to understand coherent sources of light:

  • The natural coherent sources are stars and lasers.

  • The speaker-generated sound waves are driven by electrical signals that have a constant phase difference and the same frequency.

  • Some small sources of light like soap bubbles, and butterfly wings are partially coherent on which you can see interference patterns.

  • Sunlight is a usually an incoherent source but some small areas are generally partially coherent

Method of Producing Coherent Sources

It is difficult to find a pure coherent source of light. So, you can use the following methods to generate coherent sources.

By Dividing the Wavefront

We need to divide a light wavefront into different parts to get a coherent source of lights by using several mirrors, lenses, and even prisms. Also, we can use some techniques to get coherent sources such as Fresnel’s biprism method, Lloyd’s mirror arrangement, Young’s double-slit experiment, etc.

By Dividing the Amplitude

We can create a coherent source of light by dividing the amplitude of the incoming beam into several parts. To divide the amplitude, we can use a method of partial refraction or reflection. After that, it travels through new paths to create interference.

Interference - A concept in Coherent Sources of Light

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Interference is one of the common ways to understand how coherent sources of light work. Interference is a phenomenon in which two waves of the same frequency and constant phase difference overlap with each other to form the resultant wave.

Both waves interfere with each other to create larger ripples or smaller ones depending on the exact timing of the different sets of waves.


When you view a body of water such as a river, the waves can interfere with each other to create larger or smaller waves, depending on the exact timing of different sets of waves. Each water wave will have peaks where they coincide with each other (amplitude = 1) and troughs where they coincide with each other (0 amplitude).

Types of Interference

Interference occurs whenever two waves pass through each other, or when one wave reflects off a surface and then interferes with itself. There are two main types of interference that occur, though there are many variations on these basic patterns.

They include the following:

Constructive Interference

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Constructive interference occurs when two waves overlap to produce a single wave with an amplitude that is greater than either of its component waves. The combined wave is said to be 'constructive' because it adds to, rather than subtracts from, what was there before.

An example of constructive interference can be seen when one sprinkles sand into the water, where individual sand grains add up to create larger ripples. This effect can also be observed in Young's double-slit experiment, which demonstrates how light behaves as both a particle and a wave.

Disruptive Interference

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Disruptive Interference is a phenomenon in which light from one source interferes with another coherent light source. This interference can be constructive, resulting in a brighter spot of light, or destructive, resulting in a darker spot of light. Understanding how to handle disruptive interference when working with coherent sources of light will help you build and measure sensitive detectors, like photomultiplier tubes.

Disruptive interference also occurs when we look at stars through our atmosphere. The atmospheric turbulence causes some parts of the starlight to destructively interfere with other parts, creating dark spots on its surface.


Q1. How to get a Sustained Interference?

Ans. In the interference model, we get a resultant wave when two waves are combined with each other, which is consistent with the location of maximum and minimum intensity over time called continuous and permanent intervention patterns.

To get a sustained interference we need to follow these conditions

  • Both Sources should be very narrow

  • Both sources should be coherent

  • Both sources need to have close to each other which create distinct and spacious fringes.

Q2. What are the parameters for coherent sources of light?

Ans. You must need coherent sources of light to observe the effect of optical phenomena.

A perfect coherent source should have the following conditions should

  • Same Phase and amplitude

  • Same intensity and Wavelength

  • Zero or constant phase difference

  • Same Speed and phase

Q3. Why can't two different sources form interference patterns?

Ans. Because two independent sources of light will not be the coherent sources, So they can’t be used to create an interference pattern. If we have two sources of light with different frequencies, they won’t interfere with each other because they are independent to each other.

Let’s suppose that we have a red source and a blue source. The intensities of both these sources will vary depending on their respective distances from a plane at an angle. Since their frequencies don’t match, they can never produce any interference pattern.

Q4. What are the conditions for the interference of light?

Ans. Interference is an important concept to understand coherent sources of light.

To get a constant and permanent interference pattern, a light source should have the following conditions:

  • The sources of light should have coherent

  • Should have the same intensities and amplitudes

  • The source of light should have narrow

  • The sources should produce a single wavelength

  • The distance between source and screen should have large.

Updated on 13-Oct-2022 11:19:47