Question

What are the coherent sources of light and why do we need coherent sources for the production of interference pattern ?

Answer 1

COHERENT SOURCES :
Sources of waves which emit waves of same frequency, same wavelength travelling with same speed in a medium and maintaining constant phase difference between them are called coherent sources .

Two independent sources of light are not coherent and hence can not produce interference because light beam is emitted by millions of atoms radiating independently. So the phase difference between waves from such sources fluctuates randomly many times per second. Thus any change of phase in real source will cause a simultaneous and equal change in its image.
That is with such an incoherent source, interference is not possible.