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 meant by interference of light in Physics, as per the CBSE 2025-26 syllabus?
The interference of light refers to the phenomenon where two or more coherent light waves superimpose, resulting in a redistribution of light intensity and the formation of alternating bright and dark fringes. This occurs due to the constructive and destructive interference of waves having a constant phase difference.
2. What are the essential conditions required for sustained interference of light?
For sustained interference of light, the following conditions are essential:
- The sources must emit coherent waves (constant phase difference).
- Light must have the same frequency and wavelength.
- Sources should have equal or nearly equal intensity.
- The light should be monochromatic and have the same polarization state.
- Sources should be close to each other to produce observable fringe width.
3. Explain the difference between constructive and destructive interference in the context of light.
Constructive interference occurs when the crest of one light wave falls on the crest of another, resulting in a maximum amplitude (bright fringe). Destructive interference happens when the crest of one wave coincides with the trough of another, leading to minimum amplitude (dark fringe). The pattern alternates between bright and dark regions due to these effects.
4. Describe a real-life example where the interference of light can be observed.
A classic real-life example of light interference is seen in soap bubbles, where a spectrum of colors appears. This occurs due to constructive and destructive interference between light waves reflected from the inner and outer surfaces of the thin soap film, producing varying colors depending on the film's thickness.
5. What are coherent sources in interference experiments, and why are they important?
Coherent sources emit light waves of constant phase difference, same frequency, and the same wavelength. In interference experiments, such as Young’s double-slit experiment, coherence ensures a stationary and observable interference pattern. Sources like lasers and the arrangement in Young's experiment generate coherent light necessary for sustained interference.
6. Why does interference of light not usually occur with everyday light sources like bulbs or the Sun?
Most everyday light sources, including bulbs and the Sun, emit incoherent light—this means the emitted waves have random phases and varying frequencies. Due to this randomization, the interference pattern rapidly changes in time and averages out, so no stable pattern is visible to our eyes.
7. How is Young’s double-slit experiment significant in demonstrating the interference of light?
The Young’s double-slit experiment is significant because it offers direct experimental evidence of the wave nature of light. By using two slits to create coherent sources from a single source, it shows a series of bright and dark fringes on a screen, verifying the principle of constructive and destructive interference of light waves.
8. What happens if the two sources in an interference experiment have slightly different wavelengths?
If two light sources in an interference experiment have slightly different wavelengths, the phase difference varies over time, leading to shifting and eventually washed-out fringes. The interference pattern becomes unstable and may disappear, as constant phase relation is required for sustained interference.
9. How does intensity change at points of constructive and destructive interference?
At points of constructive interference, the light intensity is at its maximum because the waves reinforce each other. At points of destructive interference, intensity is at its minimum or zero due to cancellation of waves. This creates the visible pattern of alternating bright and dark fringes.
10. Why is it necessary for the sources to be close together in an interference of light experiment?
Sources need to be close together in an interference experiment to ensure that the resulting fringes are widely spaced and easily distinguishable. If the sources are far apart, the fringe width decreases, making the pattern difficult to observe, which limits accuracy in measurements and applications.
