Question

Can two independent sources of light be coherent?

Answer 1

1. Two sources are considered to be coherent when their wavelength and frequency are the same.
2. The light from a source is produced by the excitement of atoms each of which acts independently. This means that no coherence will exist for the atoms.
3. It is for this reason that, two independent sources of light are unable to have coherence as they will never have the same phase and it is not possible to maintain the same phase difference within a similar interval of time.