CBSE Class 12-science Answered
Two light sources are said to be coherent if they are emitting light of same wavelength (or same frequency) such that the originating phase difference between them is either zero or has a fixed constant value.
Two identical and independent sodium lamps can never be coherent.In a light source , light is produced by billions of atoms under proper excitation condition and each atom acts independently of the other atoms.It means randomisation of phase and consequently no coherence between the light emitted by different atoms.The phase difference between the light rays from the two sources will not remain constant with time.Thus it is clear that two independent light sources , howsoever identical cannot behave as coherent sources.