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Interference in Physics

Interference Definition Physics

Let’s suppose that you and your friend plan out a day to meet at your favorite 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, that point is exactly the interference.


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


In Physics, we can say that interference is a phenomenon in which two waves superpose to form a resultant wave of greater, lower, or the same amplitude. Both Constructive and destructive interference result from the interaction of waves that are correlated or coherent with each other because they come from the same source. The Interference effects can be observed with all types of waves

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 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 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 security under these bravos, and this situation 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

There are two types of interference in Physics which are described as below:

  • Constructive Interference

This type of interference is  called constructive interference. Constructive interference is interference that occurs at any location along with the medium where the two interfering waves have a displacement in the same direction.

Both waves have an upward displacement as the  medium has an upward displacement that is greater than the displacement of the two interfering pulses. The constructive interference is observed mainly at  locations where the two interfering waves are displaced upward and also both interfering waves are displaced downward. 

Example

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

  • Destructive Interference

Destructive interference occurs at any location along with the medium where the two interfering waves have a displacement in the opposite direction. When  a sine pulse with a maximum displacement of +1 unit meets a sine pulse with a maximum displacement of –1 unit, destructive interference occurs. 

Constructive Interference VS Destructive Interference 

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

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 of this 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 canceling may produce no effect and that’s the point where we get the destructive interference.

Equation of Interference in Physics

Constructive Interference: When the phase difference is an even multiple of π (φ = ….., –4π, –2π, 0, 2π, 4π,……), then cos φ/2 =1, so the sum of the two waves is a wave with twice the amplitude.

 

W1+W2 = 2A cos(kx−ωt)W1+W2=2A cos (kx−ωt)


Destructive interference: When the phase difference is an odd multiple of π (φ =….., –3π, –π, 0, π, 3π, 5π,……), then cos φ/2 = 0, so the sum of the two waves will be zero.

W1+W2=0

Constructive Interference Equation

The equations for constructive interference are as follows:

           y1 = Cos (kx – t), and

           y2 = Cos Cos(kxt+π2)

Here,

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. 

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 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 a 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.

FAQs on Interference in Physics

1. State the difference between interference and diffraction of light.

Interference 

Diffraction

In interference, it happens in a way that the superposition of waves initiates 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.

2. Illustrate the concept of constructive interference in electromagnetic waves. How can we create constructive interference, illustrate an example for the same?

Constructive interference occurs when the path difference between successive crystal planes becomes 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 push the air forwards in the form of vibrations.

3. What are the properties of interference?

 The interfering waves must have the following properties:

  • The two interfering waves must have the same period and wavelength.

  • The waves must have the same amplitude or nearly the same amplitude.

  • The interfering waves must propagate in the same direction.

  • If the interfering waves are polarised. They must have the same plane of polarisation.

4.  What are maxima and minima in interference?

In interference, maxima are a point where two crests or two troughs of two different waves meet each other and, as a result, reinforce each other whereas minima in interference is a point where a crest and a trough meet together canceling out each other.

5. Can interference happen without diffraction?

Yes, in the case of thin-film interference, the phenomena of interference happen without diffraction. Thin-film interference is a natural phenomenon in which light waves reflected by the upper and lower boundaries of a thin film interfere with one another, either enhancing or reducing the reflected light.

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