Chapter-10|Wave Optics|NCERT 12th Physics:
When two or more waves travelling through a medium, superpose on each other, then a new wave is formed in which resultant displacement is equal to to the vector sum of displacement due to individual wave.
Coherent Source of Light:
Coherent source of light are those source of light which emit light waves of same wavelength, same frequency and are in same phase or having coherent phase difference.
Incoherent Source of Light:
Such sources of light, which do not emit light waves with a constant phase difference are called incoherent source of light.