Characteristics of Coherent Sources

In quantum physics, coherent sources are a major and important topic. Before getting started, we have to familiarise ourselves with the term “coherent”. In the English language dictionary it means easy to understand, or ideas that are connected and flow together smoothly. But in physics, the word “coherent” is different.

What are coherent sources?

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 to 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). 

Definitions

Coherence is a term that describes and illustrates the relationship between physical quantities in a single or several waves. This concept is very important in quantum physics. 

1.  Coherent source – When light emits waves that are two or more and have the same frequencies, the same wavelength, and zero or 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 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 state in which photons are all in step or order, or a change of phase within the beam occurs for all the photons simultaneously. Keep in mind that two separate sources can never be coherent since it is almost impossible to exactly match their frequencies and 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  

Examples of coherent source

1. Laser light – the light emitted by lasers 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 excites and generates 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. The sound wave generated from this transmitter and that wave have the same frequency level, and the phase difference is also the same. Sound waves create vibration in the air that 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 as 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. When a light wave diverges 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 a constant phase difference.