Interference, in physics, is when two waves come together. It is a phenomenon in which when light waves move in a medium, there is a change or variation in the intensity of light of these waves.

When two light waves of equal frequency travel in the same medium in the same direction, the intensity of the light changes after their superposition. This incident is called interference of light.

The intensity of light is maximum at some places and minimum or zero at other places.

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. Before getting started, we have to familiarise ourselves with the term “coherent”.

Why are Coherent Sources Essential for Observable Interference?

We need this coherent source to observe the effects of certain optical phenomena such as interference in a lab. 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. 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. This is a sufficient condition for Observable Interference. That’s why coherent sources are essential for observable interference.

Examples of coherent source

• 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.
• 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

• The waves which are produced have a constant phase difference. 
•  The waves all have the same frequency.
• Coherent waves can interfere as they have constant phase relations. 
• They also have the property which enables waves to have stationary interference.
• Coherent lights have the same frequency that they have a beam of photons.  

Interference of light

Interference is a phenomenon based on the conservation of energy, i.e., energy is neither created nor lost in it. 

This phenomenon occurs due to the superposition between two isolated waveforms moving from two phase-related sources.

In this, all the bright fringes are of equal intensity and are in increasing order of intensity. In interference, the fringes are of equal width.

The value of intensity is maximum at some points in the resultant wave and minimum at some places. On this basis, the interference of waves is divided into two parts. 

Constructive interference  

When both the waves are transmitted in the same phase and are superimposed on each other, the maximum value of intensity is attained. This is called constructive interference. In the event of interference, at the points at which the value of intensity is maximum, the phenomenon of interference occurring at those points is called constructive Interference. 

Destructive Interference

Whenever two waves move together in the same direction, and if they meet in opposite phases at any point, the resultant intensity at the point is minimum or zero. This is called Destructive Interference. 

It means that the value of the resultant intensity in Destructive Interference is minimum or zero. At any point where the value of the resultant intensity is minimum or zero, the interference at those points is called Destructive Interference. 

Conclusion

From the above discussion, we can conclude that the waves emitted from light having the same frequency, wavelength, and constant phase difference are coherent sources of light. Not all 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.