Interference of Light

VSAT 2022

What is Interference of Light?

While two or more waves superimpose on each other, there will be a modification in the energy of the light distribution. This state is known as Interference. For the interference, the waves emitted from the source should have zero phase difference or no phase difference. It is important to note that the source should emit continuous waves of the same wavelength for the same period of time. Also, both sources should be close to each other. If the waves are coherent, then the observed interference pattern is stable.


If the waves are incoherent, then the observed interference pattern is not stable and invisible.


This article explains the theory of interference of light, what are coherent sources, coherent sources and sustained interference, types of interference of light, characteristics of coherent sources, and all other information in detail.


Coherent Sources of Light and Sustained Interference

The Interference of light is a natural phenomenon. The interference of light may occur at every place and at every moment. The superimpose of two light waves with the lower or higher or same amplitude, then the resultant wave is known as interference. The interference is generally known as optical interference or light interference. As the light leaves can randomly generate from many sources. So, the light waves which are coming out of a source do not have a constant amplitude or phase, or frequency.


Interference of Light Examples

  • When the soap bubble is illuminated by a light source, it reflects a wide colour.


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  • The light generated from the incandescent bulb with a wide range of frequencies will have all the colours of the rainbow. The light coming out from the bulb was randomly spread over all the directions. Here, the frequency of the light wave near the source is maximum and will be minimum at any point away from the source. Here, it is impossible to predict the phase of the wave, so the source of the light is said to be incoherent.


What are Coherent Sources?

  • If the waves emitted from the two light sources have the same frequency and constant phase difference, then the two sources are said to be coherent sources.

  • It is possible to have the interference of light with coherent sources. Here, the randomly phased light waves will constantly produce dark and bright fringes at every point.


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  • But it cannot be noted since it occurs randomly. A point may have a dark fringe at one moment and bright fringe at the next moment. So, this phenomenon will eliminate the interference effect and the observer can visualize only an average brightness value. As an observer cannot observe, the interference is not said to be sustained interference.


Characteristics of Coherent Sources

The important characteristic of coherent sources is listed below in detail.

  • The wave generated from the sources should have a constant phase difference.

  • The waves should have a single frequency.


Example for Coherent Sources

  • Laser light is an important example of a coherent source of light. Here, the light emitted from the laser will have the same phase and frequency.

  • Another important example of coherent sources is sound waves. Here, the electrical signal generated by the sound waves will travel at the same frequency and phase.


Types of Interference

The interference of light waves is mainly classified into two types. They are given below.

  • Constructive interference

  • Destructive interference


Constructive Interference

If the crest of one wave falls on the crest of another wave, then the amplitude of the wave became maximum and it forms the constructive interference of light. Here, the resultant waves will have the same phase and the same displacement.


Destructive Interference

If the crest of one wave falls on the dip of another wave, then the amplitude of the wave becomes minimum. This phenomenon is called destructive interference. Here, the phase and displacement of the resultant wave are not the same.


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The given image shows the difference between constructive and destructive interference in detail.


Young's Double Slit Experiment

The young scientist Young demonstrated an experiment to prove the wave nature of the light. Meanwhile, he also explained the phenomenon of interference of light. Initially, he generated two coherent light sources using a single slit of diffracted light for performing Young’s double-slit experiment. Here, the generated waves had a constant phase difference. So, he utilized only a first slit to observe an interference pattern.

The most commonly used coherent sources are Laser. Because of its simulated emission property, which helps them to generate highly coherent light waves. Through this experiment, he found that even the small source of lights is also least partially coherent. That’s why observers can notice the interference patterns on the soap bubbles and the wings of butterflies showing different colours. Even though the sunlight is incoherent, the small portion of light in the small area remains are partially coherent.


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The given image shows Young's double-slit experiment, which explains the theory of interference of light and the concept behind diffraction and interference of light


Conditions for Interference of Light Waves

Conditions for the interference of the light waves are given in detail below.

  • It is important to have a coherent light source to obtain sustainable interference.

  • The light source used for the interference must be “monochromatic” in nature.

  • For interference to take place the sources must have equal amplitudes and intensities, which must be equal to produce sufficient contrast between maxima and minima.

  • The interfering sources should be near enough to produce wide fringes.

  • The coherent light source should emit the light in the same polarization state.

This article explained the characteristics, properties, types of coherent sources, and sustained interference in detail. This article also describes the diffraction and interference of light in detail.

FAQs on Interference of Light

1. What is the principle of interference of light?

Ans: When the light reflects from the inner surface of the bubble, the light must travel farther than the light reflected from the other surface. At that time, the interference effect can be observed.  The interference effect may vary with respect to the thickness of soap film and the distance travelled by light waves to reach eyes. 

2. What is an interference of light property?

Ans: The superposition of light waves produce light interference, which has two coherent sources and reinforcement of light in some regions (bright fringes) and weakness in other regions (dark fringes).  The interference fringes contain a sequence of dark and bright parallel regions. 

3. What are the advantages and disadvantages of interference of light?

Ans: The differential interference contrast technique is widely used in living specimens. This process is useful for normal staining procedures. But the interference of light techniques could not provide accurate results while examining the three-dimensional images.  

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