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Coherent & Incoherent Addition of Waves | Physics Class 12 - NEET PDF Download

You must have seen light coming out from the laser. Let us carry out a small activity. Take two needles and touch the needles on the surface of the water. Here if both the needles move with the same speed then they are said to be coherent. Let us learn more about coherent waves.

Coherent and Incoherent Addition of waves

Suppose there is a surface of the water and you take a needle and touch the surface of the water. What will happen? Yes, ripples are formed. Now if you take two needles and you touch the surface of the water with the needles. What do you think will happen?
You will see a pattern. That pattern is the interference pattern. When you touch both the needles at the surface of the water at the same time, both the needles are in the same phase. Needle 1 will produce a wave. Also, needle 2 will produce its own ripples and they will intersect with waves of the first needle.
Now, if both the needles are moving with the same velocity, the wave formed here are coherent. If the velocity of a 1st needle and 2nd needle are not steady they won’t intersect. This is because one is at a steady speed and other is at variable speed.

Coherent Waves

If the potential difference between two waves is zero or is constant w.r.t time, then the two ways are said to be coherent.

Non-coherent Waves


Coherent & Incoherent Addition of Waves | Physics Class 12 - NEET

The waves are non-coherent if the potential difference between the two ways keeps on changing. Lightbulb, study lamp are the examples of the coherent waves. They emit waves at random potential difference.

Explanation

Now let us consider there are two needles say S1 and S2 moving up and down on the surface of the water and are pointing at point P. So the path difference here is given as S1P –  S2P. Now the displacement by two needles and S1 S2 are:

y1 = A cos wt ……………… (1)
y2 = A cos wt …………….. (2)

So the resultant displacement at point P is, y =  y1  + y2. When we substitute the value of y1  and y2  we write,

y = A cos wt + A cos wt
y = 2A cos wt……………….. (3)

Now, we know the intensity is proportional to the square of the amplitude waves.

I0  ∝ A²

Where I0  is the initial intensity and  A² is the amplitude of the wave. From equation 3, we say that A = 2A. So,

I0  ∝ (2A)² or I0  ∝ 4 A²
I = 4 I0    

Now, if two needles that are Sare S2 are in the same phase, the potential difference is,

S1P  – S2P = nλ

Where n = 0, 1, 2,3 ……… and λ = the wavelength of the wave. If the two needles  S1 and S2 are vibrating at its destructive interference then, the potential difference is

S1P –  S2P = (n + 1/2) λ

Now if the potential difference of the waves is Φ then,

y1  = α cos wt

y2 = α cos wt

The individual intensity of each wave is  I0 , we get,

y =  y1  + y2
= α cos wt + α cos (wt +Φ)
y = 2 α cos(Φ/2) cos (wt + Φ/2)

Since, the intensity is I0 ∝ A²

I0 ∝ 4α² cos² (Φ/2)
I = 4 I0  cos² (Φ/2)

Well, the time-averaged value of cos²(Φt/2) is 1/2. So, the resultant intensity will be I = 2 I0 at all the points.

The document Coherent & Incoherent Addition of Waves | Physics Class 12 - NEET is a part of the NEET Course Physics Class 12.
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FAQs on Coherent & Incoherent Addition of Waves - Physics Class 12 - NEET

1. What is the difference between coherent and incoherent addition of waves?
Ans. Coherent addition of waves refers to the superposition of waves that have a constant phase relationship, resulting in constructive interference. Incoherent addition, on the other hand, involves the superposition of waves with random phase relationships, leading to destructive interference.
2. How does coherent addition of waves result in constructive interference?
Ans. When waves are added coherently, their crests and troughs align, resulting in constructive interference. This means that the amplitudes of the waves add up, resulting in a wave with a larger amplitude.
3. What happens when waves are added incoherently and why does it lead to destructive interference?
Ans. Incoherent addition of waves occurs when the waves have random phase relationships. When these waves are added, their crests and troughs do not align, leading to destructive interference. This means that the amplitudes of the waves partially or completely cancel out, resulting in a wave with a smaller amplitude.
4. What are some examples of coherent addition of waves in everyday life?
Ans. One common example of coherent addition of waves is in the production of laser light. Laser beams are created by superposing coherent waves of the same frequency and phase. Another example is interference patterns observed in double-slit experiments, where coherent light waves create a pattern of bright and dark regions.
5. How does the coherence of waves affect the quality of signals in communication systems?
Ans. In communication systems, coherence is crucial for the successful transmission and reception of signals. Coherent waves maintain their phase relationship over long distances, reducing the distortion and loss of information. Incoherent waves, on the other hand, can lead to signal degradation and interference, resulting in poor signal quality.
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