Interference in Physics

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Interference Definition Physics

Let’s suppose that you and your friend plan out a day to meet at your favourite restaurant and as the time comes you feel happy to meet each other. So, the meeting of you and your friend at commonplace is called interference in Physics. Interference Physics clearly says that when two waves coming from varying sources that are not necessarily coherent meet at a point and that point is exactly the interference. 

Here, you will learn about interference, its types and its effect on two waves and also its contrast with the diffraction of light.

Interference Physics

Now that we know the meeting point of two friends or waves coming from different places or sources is called interference. Also, we know that friendship is unity. 

Now, add another scenario by supposing that you both are army men and some intruders with their superpower guns start firing in the restaurant as for a single person it really becomes hard to fight these goons, so you both and other army men or policemen having their lunch unite or superimpose by fighting these goons together and protecting hundreds of lives.

So, here, the greater is the number of brave people, the more people’s lives are saved at the restaurant. 

In Physics, the scenario is similar, the two waves superimpose on each other to give a wave of greater amplitude; let’s see how it happens by observing the following diagram:

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So, people find secured under these bravos, and this situation clearly talks about the constructive interference of light. This context could surely give you an idea that there are types of interference and about one is already discussed above. Now, let’s talk about the types of interference.

Types of Interference

The concept of interference becomes a really interesting and fun fact when we relate this topic of Physics to real life. So there are two types of the following interference:

  • Constructive Interference of Light

  • Destructive interference of Light

Constructive Interference of Light

When we want to achieve a greater impact or source (power), we need something like constructive interference. So, constructive interference is defined as the superposition of two waves to get a greater amplitude, like we use speakers to get the greater amplitude of sounds. 


We join two or more bogies to get a big train and fetch maximum people to a particular location without having a need to manufacture two or more trains for a single location and utilize a lump sum of money for the same.

Destructive Interference of Light 

You can observe the scenario of destructive interference in cases when two waves in opposite polarity or direction superimpose on each other in a way that they cancel each other. You can see the same in the form of the following graph of pressure versus time:

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Constructive Interference VS Destructive Interference 

So, we know that the waves making a big wave is constructive interference whereas two waves cancelling each other is destructive Interference. The difference between the two types of interference is described in the form of the following graph:

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In this graph, we can see that the crest and troughs of the two waves overlap each other. The same scenario is observable when we drop a stone in water, the waves spread in the form of concentric circles, and the point where theses overlap is the interference; this is for constructive interference.

Now, talking about the destructive interference, another stone is dropped in the vicinity of these already formed concentric circular waves. So, here, the waves of another stone may cancel out the already formed waves. Now, this cancelling may produce no effect and that’s the point where we get the destructive interference.

Constructive Interference Equation

The equations for constructive interference are as follows:

           y1 = Cos (kx - t), and

           y2 = Cos (kx - t + )


ω = Frequency in per Radians

k = wave number (= 1

δ = phase difference between two waves

t = time

x = wave position in a given time ‘t’

The frequency of two waves is the same as in constructive interference, we get the superimposed wave of the same amplitude and frequency. 

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Point to Remember

In determining the interference of light, we use fringes, so in the pattern of interference, the intensity at minima is usually negligible or close to zero, which means minima is usually dark. 

Also, there is a very good contrast between the dark and bright fringe.

Interference and Diffraction of Light

Interference of Light

Interference of light takes place on the meeting of the two waves takes as they travel along with a similar medium. Besides this, the interference causes the medium to take a particular orientation; moreover, this shape is due to the whole effect of two individual waves on the medium’s particles.

Diffraction of Light

Diffraction is observable in the scenario of waves passing through an aperture spread out in the dark region like a light coming out of tunnel. In the case of diffraction, the size of the obstacle or aperture is of straight dimensions to the incident wave’s wavelength, and its occurrence is significant. Furthermore, it takes place when the traveling wavelength’s part gets shaded.

FAQ (Frequently Asked Questions)

Q 1: State the Difference Between Interference and Diffraction of Light.




In interference, it happens in a way that the superposition of waves initiate from varying wavefronts, which means that there is a phase difference between the two waves because these two waves are not coherent 

Diffraction occurs in a way that the superposition starts from different portions/areas of the same wavefronts, which  means that there is zero phase difference between the waves that come from the sources, as they are the coherent sources. 

All bright fringes in an interference pattern are of the same intensity. 

In a diffraction pattern, the intensity of each fringe goes on decreasing when they are taken away from the bright side; however, they can be brightened by bringing these near to the central bright fringe.  

Q 2: Illustrate the Concept of Constructive Interference in Electromagnetic Waves. How Can We Create Constructive Interference, Illustrate an Example for the Same?

Ans:  Constructive interference occurs when the path difference between successive crystal planes become equivalent to an integral number of wavelengths of the electromagnetic radiation or EM radiations. For constructive interference, the phase difference between two waves is an even multiple of π or 180°.

We can create constructive interference by using two loudspeakers. If we place two speakers side-by-side, align them in the same direction, and play the same frequency sound, we get constructive interference. As we know, sound means vibrations that travel through the medium called air, so both speakers named speaker 1 and speaker 2 pushes the air forwards in the form of vibrations.