Coherent Sources

In Physics, interference is a phenomenon where two waves form a resultant wave either of high, low, or having the same amplitude.


At any place or moment, you can expect interference but we don't see everywhere interference patterns as most of the sources generate light waves randomly. It means the light waves coming out of the source do not have a constant frequency, phase, or amplitude.


For example, a wide range of frequencies of light, like the colors of the rainbow, is generated by incandescent bulbs. The light out of the bulb randomly gets generated everywhere and in all directions. It means that the starting point of the wave generated may be minimum, maximum, or any point in between. In which the wave will start cannot be predicted; such a source is said to be incoherent. 


What are Coherent Sources?

If the sources have zero or constant phase difference and the same frequency, then the two sources are considered to be coherent. Most of the light sources around us like the bulb, sun, candle, etc. are a combination of a multitude of incoherent sources of light. For coherent sources, the laser is an example, i.e., multiple essential sources inside the laser are phase-locked.  


Coherent Sources of Light Meaning

To observe the effects of certain optical phenomena like interference in a lab, you will need coherent sources of light. Two equivalent slits lighted by a laser beam can be said to be two coherent point sources.


Further, two sources of light are said to be coherent when they give light waves of the same

  1. Phase and Amplitude

  2. Wavelength and constant phase difference

  3. Wavelength and Intensity

  4. Speed and Phase


Characteristics of Coherent Sources

Coherent sources have the following features:

  1. The waves created have a constant phase difference

  2. The waves are of a single frequency


Coherent Source Example

  • An example of a coherent source of light is the laser light. The emitted light from the laser has the same phase and frequency.

  • Another example of coherent sources is the sound waves. At the same frequency and phase, the electrical signals travel from the sound waves. 


What is Interference?

Interference occurs when two or more waves of the same wavelength and frequency join each other.


Based upon the principle of superposition of waves, according to, when a number of wave disturbances pass through a given point in a medium, the resultant disturbance created at that point is the vector sum of the disturbance due to the individual waves. 


Definition of Interference of Light 

When two light waves from different coherent sources are combined, the energy distribution due to one wave is disturbed by the other. This change in light energy distribution is called interference of light, due to the superposition of the two light waves.


Types of Interference

Constructive interference and Destructive interference are the two types of interference of light waves depending upon the waves collided.

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Constructive Interference: When two waves of similar frequency and wavelength collide, they will collide in such a way that the crest of the first wave collides with the crest of the second-wave and similarly the trough of the first wave collides with that of the second wave. In this case, the troughs and crest of both waves will add up to form a wave having equal amplitude. The sum of both the waves is known as Constructive Interference. 

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Destructive Interference: When both waves collide in such a way that the crest of the first wave collides with the trough of the second wave and the trough of the first wave collides with the crest of the second wave. Then, the trough and crest of both waves will cancel each other to form a wave. 


The formed wave will be equal to the difference in the amplitude of both the waves. If the amplitude of both waves is the same, then after the interference, the amplitude will be zero, this is known as Destructive Interference.

FAQ (Frequently Asked Questions)

1. What is Sustained Interference?

Ans Interference models that are consistent with locations of maximum and minimum intensity over time are called continuous or permanent intervention patterns.

The conditions for the creation of sustained interference may be stated as :

  1. Two sources should be coherent

  2. Two sources should be very narrow

  3. The sources should be close to each other to form distinct and spacious fringes. 

The interference that is not short-lived means the sustained interference of light (generated by random variations).


A coherent source of monochromatic light (single wavelength) is needed for a double-slit experiment.


The idea is that you need a continuous phase association between light rays to perceive a sustained interference pattern.

2.  What are Coherent Sources for Sustained Interference?

Ans.  All EM waves create interference patterns. Coherent sources create stationary patterns that can be examined more easily. A laser is an obvious solution if you wish to have a visible light source. At other wavelengths, the radio transmitter feeds multiple antennas at a distance creating similar interference patterns on a very large scale.

3. What are the Conditions for the Interference of Light.

Ans  There are following conditions for interference of light waves: the source of the waves must be constant, which emits similar waves with a constant phase difference. The wave must be monochromatic –they must have a single wavelength.

Also, for constant interference to happen, the following conditions are required:

  1. Coherent sources of light 

  2. The intensities and amplitudes must be nearly equal to produce enough contrast between minimum and maximum.

  3. To be considered as a point source of light, the source should be small enough.

  4. To produce wide fringes, the interfering sources must be near to each other

  5. To produce wide fringes, the distance between the screen and source and screen should be large

  6. The sources must produce light in the same state of polarization.

  7. The sources must have a single wavelength.

4. What are the Conditions for Constructive Interference?

These are: 

  1.  Numerous waves should be coherent,

  2. The multiple waves should have a similar wavelength, and 

  3. The waves should be polarized. Some say the light must be constant but the interference only occurs with coherent light.