CBSE Class 12-science Answered

Two essential conditions for obtaining two coherent lights are as follows.
(i) Using single source of light : In this case, any phase change in one is simultaneously accompanied by equal phase change in the other.
(ii) Two lights should be monochromatic in order to prohibit them from overlapping with each other and making their own set of interference fringes.

(iii) The path difference between light waves from two sources should be small.If the path difference is large then there is intermixing of the two at every point, resulting in uniform illumination.