Question

Can two independent sources of light be coherent?

Answer 1

Hint: The condition for making coherent sources is that the phase difference of the waves of the two sources should be constant. To answer the question, the primary concern will be if the two independent sources can have a constant phase difference during their whole journey. That means if the phase changes for one wave there must be a change in the other wave in the same amount.

Complete step-by-step solution:
Two sources are said to be coherent if they have the same wavelength as well as the same frequency. Therefore, the sources can maintain the phase difference of their waves being constant or zero. That means, from the initial position the waves must have the same phase or they have to keep any difference in phases during their whole journey in a fixed value.
In a source of light, light is produced by a large number of excited atoms that act independently or can be said in a different manner from each other. Therefore, there will be no coherence for the different atoms. So, the two different or independent light sources can't have the same phase and also can not maintain the same phase difference with the interval of time.
Hence, two independent sources of light can not be coherent.

Note: Some essential conditions for making the coherent sources are:
(i) A single source of light has to be used because any phase change in one is simultaneously obtained by a similar phase change in the other.
(ii) Two monochromatic lights should be used to resist them from overlapping. thus they can not make their own set of interference fringes.
(iii) The path difference between the waves of the two light sources has to be small to prevent the intermixing of the two at every point. because it causes uniform illumination.