(a)Two narrow sources of light are said to be coherent if they emit waves having. the same wavelenght (or frequency), the same amplitude, and. a constant phase relation between them.

Two sources of emitting light waves of same frequency or wavelength and of a stable phase difference are required to see interference pattern and we can obtain such nature of light waves from coherent source. So, we require coherent sources to produce the interference of light.

(b) Characteristic features are as follows:1) IM Young's slit experiment we get fringes of same intensity while in diffraction pattern we get fringes with centre fringe of maximum intensity and others are of decreasing intensity.2) In young's experiment all the fringes are at euqidistant while in diffraction pattern they are not at same distance.3) In diffarction complete interference takes place when path length is an intregal multiple while in young's it is multiple of half integral multiple.