Coherent Sources

In quantum physics, coherent sources are a major and important topic. Here we discuss and explore this topic and understand it very closely. When we talk about coherent sources, we keep thinking about the word “ coherent”. Before getting started, we had to familiarize ourselves with the term “coherent”.

The English language dictionary means easy to understand, or ideas are connected and flow together smoothly.

But here in physics, the word coherent is different.

Coherence is a correlation between phase difference, frequency and amplitude of different waves or the same wave.

As you know, light emits or produces waves. When these waves of light have the same frequencies and waveform and the phase difference between them is constant, they are said to be coherent sources of light. 

Terminology

Now, you may get confused between the terms used above: phase difference, frequencies, waveforms. 

To clear your confusion, here are some basic definitions that help you understand the keywords very easily and effectively. 

 Phase difference – it is the path difference between two waves, and it is denoted by Δ x. In other words, the time interval by which a wave leads by or lags by another wave.

Frequency – The number of waves passing through a given point in one second. It is denoted by (f) and measured in hertz (Hz). 

Body

Coherence is a term that describes and illustrates the relationship between physical quantities in a single or several waves. 

This concept is very much important in quantum physics. 

DEFINITIONS 

1.  Coherent source– When light emits waves that are two or several, having the same frequencies, the same wavelength, and have zero constant phase difference and preferably the same amplitude. 

2.  Incoherent source – When light emits waves that have random frequencies and phase differences. Photons with the same frequency and wavelengths out of phase are not seen in incoherent sources.

How can a coherent source be produced? 

A coherent source can be formed by dividing the amplitude of an entering light into two. Since the divided light originates from the same source, their frequencies are the same and phase difference is constant. This process can be done by the process of partial reflection and refraction. Furthermore, these divided parts further meet with each other to create interference. 

Coherent light is the light when photons are all in step or order, or a change of phase within the beam occurs for all the photons simultaneously.

You should know, 

Two separate sources can never be coherent since it is almost impossible to exactly match their frequencies and hence keep phase difference constant.

Two sources of light are said to be coherent when the waves of light have the same 

– phase difference and amplitude 

– wavelength and constant phase

– wavelength and intensity 

– speed and phase  

INTERFERENCE OF LIGHT WAVES 

Interference is the addition in the mathematical sense of wave function. 

Here the question arises, How does interference occur? 

In other words, “ interference occurs when two or more waves of light from different coherent sources are combined or join each other, the energy distribution due to one wave is disturbed by another. This change in the distribution of light energy is called interference of light. 

Type of interference 

There are two types of interference: constructive and the second one is destructive. 

• Constructive interference– when waves are at the same place simultaneously, then the amplitude of waves collide together i.e they have a multiple of 2π as phase difference. When the two waves pass each other, interference occurs. 

To start exploring the implication of the above statement, let’s consider two waves with the same frequencies travelling in the same direction. 

When we add these two waves, we get the resultant new wave, and that wave is more significant than the original wave and has greater amplitude. This kind of situation is called constructive interference. In simple words, when the two waves pass each other and the maxima of these two waves are added, then the amplitude of the new wave is equal to the sum of the original waves.

• Destructive interference – In this, waves of light collide in such a way that one wave is up. The second wave down means the crest of the first wave collides with the trough of the second wave, and the trough of the first wave collides with the crest of the second wave, then the trough and crest of both waves will cancel each other to form a wave. The produced wave will have an amplitude equal to the difference between the two waves. When the 180 degrees of maxima is out of phase between two waves, they are called destructive interference. The resulting wave has zero amplitude if both waves have the same amplitude individually.

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Examples of coherent source

1. Laser light – the light emitted by laser light has the same frequency and phase and constant phase difference. That’s why they are said to be coherent sources. The light has a photon, which will excite and generate a wave. This wave is said to be a coherent wave.

2. Sound wave – the electrical signals from the sound wave have the same frequency and phase. E.g. radio transmitter. As we all know about the radio transmitter, the sound wave generated from this transmitter and that wave have the same frequency level, and the phase difference is also the same. As we are familiar with sound waves, the sound waves create vibration in the air. It is a longitudinal wave. Their directions are parallel to the direction of the wave that is moving.  

Characteristics of coherent sources

1. The waves which are produced have a constant phase difference. 

2.  The waves all have the same frequency.

3. Coherent waves can interfere as they have constant phase relations. 

4. They also have the property which enables waves to have stationary interference.

5. Coherent lights have the same frequency that they have a beam of photons.  

Conclusion

From the above discussion, we can conclude that the waves emitted from light having the same frequency, the same wavelength, and constant phase difference are coherent sources of light. Not all the light waves or sound waves are coherent. We can also conclude from this topic that coherent waves have constant phase relations. Coherent sources are necessary to ensure the position of maxima and minima do not change with time as a new intensity of light produces a sustainable interference. And when a light wave is diverging from the point source, the wavefront is spherical. It is also necessary to describe all the relations between a single wave or several waves. Also, the most important thing to keep in mind is that two independent sources will never be coherent because independent sources never have the same frequency nor have a constant phase difference.