question_answer

What is meant by coherent sources of light? Can two identical and independent sodium lamps act as coherent sources? Give reason for your answer.                   

Answer:

                Two light sources are said to be coherent if they continuously emit light waves of same frequency (or wavelength) with a zero or constant phase difference between them. Two independent sources cannot act as coherent sources. The emission of light in them is due to millions of atoms in which electrons jump from higher to lower orbit. The process occurs in about \[{{10}^{-8}}\]s. Thus phase difference can remain constant for about \[{{10}^{-8}}\]s only i.e., phase changes \[\text{1}{{0}^{\text{8}}}\] times in one second. Such rapid changes in the positions of maxima and minima cannot be detected by our eyes. The interference pattern is lost and almost a uniform illumination is seen on the screen.