I think the light from the light-bulb is not coherent because it's random, right?
Only the light from lasers is coherent. It means that the stream of photons that constitute a beam of light have their vibrations all in step with each other. Think of a series of sine waves. If the waves all move up at the same time, they will also move down at the same time - they are in step, they are coherent. Light from the Sun, light from a light bulb, consists of photons that are not in step. Coherence can be forced on very weak beams of light by letting photons through a single slit, whereupon the light diffracts outwards and if then passed through twin slits will form two coherent beams which can then form a regular interference pattern. And its forming interference patterns that you want coherent light for.
in coherent light the phase of all the light rays is same or differs by a constant phase angle. whereas in ordinary light the phase angle of rays are random. eg: of coherent light is laser and that of ordinary light is a bulb in your home
