NCERT · Class 12 · Physics · Wave OpticsWhat are coherent sources of light?

Coherent sources of light are defined as those sources which continuously emit light waves of the same frequency, same wavelength, and have either a zero phase difference or a constant phase difference that does not change with time. Ordinary independent sources of light cannot be coherent because light is emitted by individual atoms undergoing spontaneous transitions lasting for very short durations of about 10 to the power -8 seconds. Hence, special optical devices are used to obtain coherent sources.